Add an option to keep the ROM cached after romstage

Add an option to keep the ROM cached after romstage

Change-Id: I05f1cbd33f0cb7d80ec90c636d1607774b4a74ef
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/739
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>