selfboot: fix bug in valid_area()

selfboot: fix bug in valid_area()

valid_area will accept a region as valid for the payload if only a part
of coreboot fits in that region. This means if a payload reaches into a
neighboring RESERVED region, coreboot would not care and happily
overwrite that region, as long as the payload also writes to some RAM.

Change-Id: Ie263f83be18009b01a31c71e7285c998747d097f
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/425
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>