Add a separate library for initializing px4 userspace in protected build

The px4_userspace_init function in this library is called from userspace
entrypoint before starting NSH

Signed-off-by: Jukka Laitinen <jukkax@ssrc.tii.ae>

platforms/nuttx/src/px4/common/px4_userspace_init.cpp

@@ -0,0 +1,16 @@
+#include <drivers/drv_hrt.h>
+#include <lib/parameters/param.h>
+#include <px4_platform_common/px4_work_queue/WorkQueueManager.hpp>
+#include <uORB/uORB.h>
+#include <sys/boardctl.h>
+
+extern "C" void px4_userspace_init(void)
+{
+	hrt_init();
+
+	param_init();
+
+	px4::WorkQueueManagerStart();
+
+	uorb_start();
+}