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/nuttx/cmake/px4_impl_os.cmake

@@ -64,10 +64,19 @@ function(px4_os_add_flags)
 		${PX4_SOURCE_DIR}/platforms/nuttx/NuttX/apps/include
 	)
 
-	# prevent using the toolchain's std c++ library
-	add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-nostdinc++>)
 
-	add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-fno-sized-deallocation>)
+	set(cxx_flags)
+	list(APPEND cxx_flags
+		-fno-exceptions
+		-fno-rtti
+		-fno-sized-deallocation
+		-fno-threadsafe-statics
+		-nostdinc++ # prevent using the toolchain's std c++ library
+	)
+
+	foreach(flag ${cxx_flags})
+		add_compile_options($<$<COMPILE_LANGUAGE:CXX>:${flag}>)
+	endforeach()
 
 	add_compile_options($<$<COMPILE_LANGUAGE:C>:-Wbad-function-cast>)