cmake: avoid GLOB for *.msg files and use an explicit .msg file enumeration

This makes sure that adding & removing of .msg files is handled properly
by the build system.

CMakeLists.txt

@@ -295,7 +295,7 @@ add_definitions(${definitions})
 #=============================================================================
 # source code generation
 #
-file(GLOB msg_files msg/*.msg)
+add_subdirectory(msg)
 px4_generate_messages(TARGET msg_gen
 	MSG_FILES ${msg_files}
 	OS ${OS}