create PX4 platform layer initialization helper (#11269)

- starts requirements for PX4 modules (hrt, param, etc)

src/systemcmds/motor_ramp/CMakeLists.txt

@@ -38,6 +38,4 @@ px4_add_module(
         -Wno-write-strings
     SRCS
         motor_ramp.cpp
-    DEPENDS
-        platforms__common
     )

src/systemcmds/shutdown/CMakeLists.txt

@@ -37,7 +37,4 @@ px4_add_module(
 	COMPILE_FLAGS
 	SRCS
 		shutdown.c
-	DEPENDS
-		platforms__common
 	)
-# vim: set noet ft=cmake fenc=utf-8 ff=unix :