Makefile: Create empty <config.h> if this header file is not needed by board
Patch creates empty <config.h> if this header is not needed by board.
This will allow to remove many empty config.h files from boards.
Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>