Rework bootblock size handling:

Rework bootblock size handling:
- don't pretend to create a bootblock as large
  as the ROM in Kconfig (it's 64k at most)
- don't pretend to accept a bootblocksize value
  in cbfstool create (it ignored it)
- patch up the build systems to keep it working

Signed-off-by: Patrick Georgi <patrick.georgi@coresystems.de>
Acked-by: Myles Watson <mylesgw@gmail.com>
Acked-by: Peter Stuge <peter@stuge.se>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@4934 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1