initial minimal PX4_ROS2 platform and px4_ros2_default build (#20689)

- new ROS2 platform in PX4 intended for creating configs that build and run entirely in ROS2
 - PX4_CONFIG defaults to px4_ros2_default if no config specified and in a colcon workspace with ROS_VERSION=2
 - currently doesn't do much other than allow you to build px4 msgs interface package

platforms/common/work_queue/CMakeLists.txt

@@ -32,9 +32,9 @@
 ############################################################################
 
 # nuttx uses internal work queue currently
-if (NOT "${PX4_PLATFORM}" MATCHES "nuttx")
+if(NOT "${PX4_PLATFORM}" MATCHES "nuttx")
 
-	add_library(work_queue
+	add_library(work_queue STATIC
 		dq_addlast.c
 		dq_rem.c
 		dq_remfirst.c

platforms/common/work_queue/work_lock.c

@@ -35,6 +35,7 @@
 #include <stdio.h>
 #include "work_lock.h"
 
+#include <px4_platform_common/sem.h>
 
 extern px4_sem_t _work_lock[];