cbfstool: provide structure to linux payload builder

This change started with tracking down a bug where the trampoline
size was not being taken into account for sizing the output buffer
leading to a heap corruption.  I was having a hard time keeping
track of what num_segments actually tracked as well as what parts
were being placed in the output buffer. Here's my attempt at
hopefully providing more clarity.

This change doesn't crash when adding a bzImage:
$ dd if=/dev/zero of=bb.bin bs=64 count=1
$ ./cbfstool tmp.rom create -s 4M -B bb.bin -m x86 -a 64
$ ./cbfstool tmp.rom add-payload -f ~/Downloads/bzImage -C "1" -n
"fallback"/payload

Change-Id: Ib1de1ddfec3c7102facffc5815c52b340fcdc628
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/5408
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Marc Jones <marc.jones@se-eng.com>