Initial Linux support including execution shell

Uncomment the following line in setup.mk and comment out the line above
to enable the Linux build.

   export PX4_TARGET_OS	 = linux

The build uses the clang compiler by default. The final bundled executable
is mainapp located in:

   Build/linux_default.build/mainapp

When you run mainapp it will provide a list of the built-in apps. You can
type in the commands to run such as:

   hello_main start

Because the Linux build is threaded and does not support tasks or processes,
it cannot call errx, exit() _exit(), etc. It also requires unique scoped
variables to test if a thread is running or if an application should exit.
The px4::AppMgr class was added in px4_app.h for this purpose. The
hello sample app demonstrates how this is used.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/platforms/nuttx/px4_layer/module.mk

@@ -0,0 +1,40 @@
+############################################################################
+#
+#   Copyright (c) 2015 Mark Charlebois. All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+#
+# 1. Redistributions of source code must retain the above copyright
+#    notice, this list of conditions and the following disclaimer.
+# 2. Redistributions in binary form must reproduce the above copyright
+#    notice, this list of conditions and the following disclaimer in
+#    the documentation and/or other materials provided with the
+#    distribution.
+# 3. Neither the name PX4 nor the names of its contributors may be
+#    used to endorse or promote products derived from this software
+#    without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+# FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+# COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+# BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+# OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+# AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+# ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+# POSSIBILITY OF SUCH DAMAGE.
+#
+############################################################################
+
+#
+# NuttX porting layer
+#
+
+SRCS		 = 	px4_nuttx_tasks.c
+
+MAXOPTIMIZATION	 = -Os

src/platforms/nuttx/px4_layer/px4_nuttx_tasks.c

@@ -0,0 +1,118 @@
+/****************************************************************************
+ *
+ *   Copyright (C) 2012 PX4 Development Team. All rights reserved.
+ *   Author: @author Lorenz Meier <lm@inf.ethz.ch>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name PX4 nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+/**
+ * @file px4_nuttx_tasks.c
+ * Implementation of existing task API for NuttX
+ */
+
+#include <nuttx/config.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <fcntl.h>
+#include <sched.h>
+#include <signal.h>
+#include <unistd.h>
+#include <float.h>
+#include <string.h>
+
+#include <sys/stat.h>
+#include <sys/types.h>
+
+#include <stm32_pwr.h>
+
+#include <systemlib/systemlib.h>
+
+// Didn't seem right to include up_internal.h, so direct extern instead.
+extern void up_systemreset(void) noreturn_function;
+
+void
+px4_systemreset(bool to_bootloader)
+{
+	if (to_bootloader) {
+		stm32_pwr_enablebkp();
+
+		/* XXX wow, this is evil - write a magic number into backup register zero */
+		*(uint32_t *)0x40002850 = 0xb007b007;
+	}
+	up_systemreset();
+
+	/* lock up here */
+	while(true);
+}
+
+static void kill_task(FAR struct tcb_s *tcb, FAR void *arg);
+
+void px4_killall()
+{
+//	printf("Sending SIGUSR1 to all processes now\n");
+
+	/* iterate through all tasks and send kill signal */
+	sched_foreach(kill_task, NULL);
+}
+
+static void kill_task(FAR struct tcb_s *tcb, FAR void *arg)
+{
+	kill(tcb->pid, SIGUSR1);
+}
+
+int px4_task_spawn_cmd(const char *name, int scheduler, int priority, int stack_size, main_t entry, char * const argv[])
+{
+	int pid;
+
+	sched_lock();
+
+	/* create the task */
+	pid = task_create(name, priority, stack_size, entry, argv);
+
+	if (pid > 0) {
+
+		/* configure the scheduler */
+		struct sched_param param;
+
+		param.sched_priority = priority;
+		sched_setscheduler(pid, scheduler, &param);
+
+		/* XXX do any other private task accounting here before the task starts */
+	}
+
+	sched_unlock();
+
+	return pid;
+}
+
+int px4_task_delete(int pid)
+{
+	return task_delete(pid);
+}