Allow user to specify the size of a newly created cbfs image

Allow user to specify the size of a newly created cbfs image
to be stated in kilobytes or megabytes. Usage is
cbfstool coreboot.rom create 1048576 coreboot.bootblock
cbfstool coreboot.rom create 1024k coreboot.bootblock
cbfstool coreboot.rom create 1m coreboot.bootblock
to get an 1048576 bytes = 1024kb = 1mb image.

Kconfig also uses this instead of calculating bytes from kilobytes itself.

Signed-off-by: Patrick Georgi <patrick.georgi@coresystems.de>
Acked-by: Peter Stuge <peter@stuge.se>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@4987 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1