build system: Mark clean-for-update phony

build system hygiene, not known if this actually matters.

Change-Id: Ic800a2acecff123fc2055047fab67df107ac43ab
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/2356
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins)
1 file changed