payloads/external/GRUB2: Only rebuild on config changes

Change-Id: I376eea9a3e02b03010fc5c5a05199ea7b2813220
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/20991
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>

payloads/external/GRUB2/Makefile

@@ -22,7 +22,7 @@ checkout:
 			git branch -f $(NAME-y) $(TAG-y) && \
 			git checkout $(NAME-y) || true
 
-config: checkout
+grub2/build/config.h: $(CONFIG_DEP) | checkout
 	echo "    CONFIG     GRUB2 $(NAME-y)"
 	rm -rf grub2/build
 	mkdir grub2/build
@@ -32,6 +32,8 @@ config: checkout
 	CFLAGS=-O2 TARGET_CFLAGS=-Os \
 	--with-platform=coreboot --enable-boot-time --disable-werror
 
+config: grub2/build/config.h checkout
+
 grub2: config
 	echo "    MAKE       GRUB2 $(NAME-y)"
 	$(MAKE) -C grub2/build