cmake: px4_add_module improve unity build dependencies to avoid unnecessary work

cmake/px4_add_module.cmake

@@ -107,7 +107,7 @@ function(px4_add_module)
 		# unity build
 		add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${MODULE}_unity.cpp
 			COMMAND cat ${SRCS} > ${CMAKE_CURRENT_BINARY_DIR}/${MODULE}_unity.cpp
-			DEPENDS ${MODULE}_original ${DEPENDS} ${SRCS}
+			DEPENDS ${SRCS}
 			COMMENT "${MODULE} merging source"
 			WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
 			)
@@ -115,6 +115,7 @@ function(px4_add_module)
 
 		add_library(${MODULE} STATIC EXCLUDE_FROM_ALL ${CMAKE_CURRENT_BINARY_DIR}/${MODULE}_unity.cpp)
 		target_include_directories(${MODULE} PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
+		add_dependencies(${MODULE} ${MODULE}_original) # build standalone module first to get clean compile errors
 
 		if(COMPILE_FLAGS)
 			target_compile_options(${MODULE}_original PRIVATE ${COMPILE_FLAGS})