cbfstool: fix option parsing

"cbfstool create -B bootblock -s size" (in this order)
would break bootblock selection.

Change-Id: I9a9f5660827c8bf60dae81b519c6f026f3aaa0f3
Found-by: Coverity Scan
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/6564
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins)