cmake remove circular linking and reorganize

- px4_add_module now requires MAIN
 - px4_add_library doesn't automatically link

platforms/posix/cmake/px4_impl_os.cmake

@@ -164,21 +164,19 @@ function(px4_os_add_flags)
 		LINK_DIRS ${LINK_DIRS}
 		DEFINITIONS ${DEFINITIONS})
 
-        set(added_include_dirs
-		platforms/posix/include
-		)
-
-	# This block sets added_definitions and added_cxx_flags.
 	set(added_c_flags)
 	set(added_cxx_flags)
 	set(added_exe_linker_flags)
-	set(added_definitions
+
+	add_definitions(
 		-D__PX4_POSIX
 		-Dnoreturn_function=__attribute__\(\(noreturn\)\)
 		)
+		
+	include_directories(platforms/posix/include)
 
 	if(UNIX AND APPLE)
-		list(APPEND added_definitions
+		add_definitions(
 			-D__PX4_DARWIN
 			-D__DF_DARWIN
 			)
@@ -191,7 +189,7 @@ function(px4_os_add_flags)
 		STRING(REGEX MATCH "[0-9]+" DARWIN_VERSION ${DARWIN_VERSION})
 		# message(STATUS "PX4 Darwin Version: ${DARWIN_VERSION}")
 		if (DARWIN_VERSION LESS 16)
-			list(APPEND added_definitions
+			add_definitions(
 				-DCLOCK_MONOTONIC=1
 				-DCLOCK_REALTIME=0
 				-D__PX4_APPLE_LEGACY
@@ -199,14 +197,14 @@ function(px4_os_add_flags)
 		endif()
 
 	elseif(CYGWIN)
-		list(APPEND added_definitions
+		add_definitions(
 			-D__PX4_CYGWIN
 			-D_GNU_SOURCE
 			-D__USE_LINUX_IOCTL_DEFS
 			-U __CUSTOM_FILE_IO__
 			)
 	else()
-		list(APPEND added_definitions
+		add_definitions(
 			-D__PX4_LINUX
 			-D__DF_LINUX
 			)
@@ -316,5 +314,6 @@ function(px4_os_prebuild_targets)
 			REQUIRED OUT BOARD
 			ARGN ${ARGN})
 
-	add_custom_target(${OUT} DEPENDS uorb_headers)
+	add_library(${OUT} INTERFACE)
+	add_dependencies(${OUT} DEPENDS uorb_headers)
 endfunction()