split CBFS support into shared core and extended functions

split CBFS support into shared core and extended functions

The core is data structures and basic file finding capabilities,
while option ROM handling, and loading stages and payloads is
"extended".

The core is rewritten to be BSD-l (its header already was), so
can be copied to libpayload verbatim.
It's also more robust in finding files in corrupted images, eg.
after partial erase or update.

Change-Id: Ic6923debf8bdf3c67c75746d3b31f3addab3dd74
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/114
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>