Makefile: clean up variables

- replace $(LIBS) by $(LDFLAGS)
- use override to handle CPPFLAGS, CFLAGS, LDFLAGS
    This allows to append flags to the users input.
- remove unused $(DIFF)

Change-Id: I1c9e869377677d624469af1ee9ece9a28fc3b559
Signed-off-by: Thomas Heijligen <thomas.heijligen@secunet.com>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/59230
Original-Reviewed-by: Nico Huber <nico.h@gmx.de>
Original-Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/72282
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
1 file changed