cbfstool: add offset parameter to create command

cbfstool: add offset parameter to create command

CBFS allows coreboot rom images that are only partially covered
by the filesystem itself. The intention of this feature was to
allow EC / ME / IMC firmware to be inserted easily at the beginning
of the image. However, this was never implemented in cbfstool.

This patch implements an additional parameter for cbfstool.

If you call cbfstool like this:
cbfstool coreboot.rom create 8192K bootblock.bin 64 0x700000
it will now create an 8M image with CBFS covering the last 1M of
that image.

Test:
     cbfstool coreboot.rom create 8192K bootblock.bin 64 0x700000
     creates an 8M image that is 7M of 0xff and 1M of CBFS.

Change-Id: I5c016b4bf32433f160b43f4df2dd768276f4c70b
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/1708
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>