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/linux/px4_layer/px4_linux_tasks.c

@@ -0,0 +1,186 @@
+/****************************************************************************
+ *
+ *   Copyright (C) 2015 Mark Charlebois. All rights reserved.
+ *   Author: @author Mark Charlebois <charlebm#gmail.com>
+ *
+ * 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_linux_tasks.c
+ * Implementation of existing task API for Linux
+ */
+
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <stdbool.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 <systemlib/systemlib.h>
+
+#include <px4_tasks.h>
+
+#define MAX_CMD_LEN 100
+
+#define PX4_MAX_TASKS 100
+static pthread_t taskmap[PX4_MAX_TASKS] = {};
+
+typedef struct 
+{
+	px4_main_t entry;
+	int argc;
+	char *argv[];
+	// strings are allocated after the 
+} pthdata_t;
+
+void entry_adapter ( void *ptr );
+void entry_adapter ( void *ptr )
+{
+	pthdata_t *data;            
+	data = (pthdata_t *) ptr;  
+
+	data->entry(data->argc, data->argv);
+	free(ptr);
+	pthread_exit(0); 
+} 
+
+void
+px4_systemreset(bool to_bootloader)
+{
+	printf("Called px4_system_reset\n");
+}
+
+px4_task_t px4_task_spawn_cmd(const char *name, int scheduler, int priority, int stack_size, px4_main_t entry, char * const argv[])
+{
+	int rv = 0;
+	int argc = 0;
+	int i;
+	unsigned int len = 0;
+	unsigned long offset;
+	unsigned long structsize;
+	char * p = (char *)argv;
+
+        px4_task_t task;
+
+	// Calculate argc
+	while (p != (char *)0) {
+		p = argv[argc];
+		if (p == (char *)0)
+			break;
+		++argc;
+		len += strlen(p)+1;
+	}
+        structsize = sizeof(pthdata_t)+(argc+1)*sizeof(char *);
+	pthdata_t *taskdata;
+    
+	// not safe to pass stack data to the thread creation
+	taskdata = (pthdata_t *)malloc(structsize+len);
+	offset = ((unsigned long)taskdata)+structsize;
+
+    	taskdata->entry = entry;
+	taskdata->argc = argc;
+
+	for (i=0; i<argc; i++) {
+		printf("arg %d %s\n", i, argv[i]);
+		taskdata->argv[i] = (char *)offset;
+		strcpy((char *)offset, argv[i]);
+		offset+=strlen(argv[i])+1;
+	}
+	// Must add NULL at end of argv
+	taskdata->argv[argc] = (char *)0;
+
+	//printf("Called px4_task_spawn_cmd\n");
+	// FIXME - add handling for scheduler and priority
+        rv = pthread_create (&task, NULL, (void *)&entry_adapter, (void *) taskdata);
+	//printf("pthread_create task=%d rv=%d\n",(int)task, rv);
+
+	for (i=0; i<PX4_MAX_TASKS; ++i) {
+		// FIXME - precludes pthread task to have an ID of 0
+		if (taskmap[i] == 0) {
+			taskmap[i] = task;
+			break;
+		}
+	}
+	if (i>=PX4_MAX_TASKS) {
+		return -ENOSPC;
+	}
+        return i;
+}
+
+int px4_task_delete(px4_task_t id)
+{
+	int rv = 0;
+	int i; 
+	pthread_t pid;
+	//printf("Called px4_task_delete\n");
+
+	// Get pthread ID from the opaque ID
+	for (i=0; i<PX4_MAX_TASKS; ++i) {
+		// FIXME - precludes pthread task to have an ID of 0
+		if (taskmap[i] == id) {
+			pid = taskmap[i];
+			break;
+		}
+	}
+	if (i>=PX4_MAX_TASKS) {
+		return -EINVAL;
+	}
+	// If current thread then exit, otherwise cancel
+        if (pthread_self() == pid) {
+		pthread_exit(0);
+	} else {
+		rv = pthread_cancel(pid);
+	}
+
+	taskmap[id] = 0;
+
+	return rv;
+}
+
+void px4_killall(void)
+{
+	//printf("Called px4_killall\n");
+	for (int i=0; i<PX4_MAX_TASKS; ++i) {
+		// FIXME - precludes pthread task to have an ID of 0
+		if (taskmap[i] != 0) {
+			px4_task_delete(i);
+		}
+	}
+}
+