Fix coreboot updates

The rule to prepare a new coreboot.pre1 was ignored in the
"update image" scenario because a perfectly fine file exists.
Mark it phony to fix it.

Change-Id: Ie7f8b36b71015a593958cd6e19602bad6b854320
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/351
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>