A new CBFS API is introduced to allow making CBFS access
easier for providing multiple CBFS sources. That is achieved
by decoupling the cbfs source from a CBFS file. A CBFS
source is described by a descriptor. It contains the necessary
properties for walking a CBFS to locate a file. The CBFS
file is then decoupled from the CBFS descriptor in that it's
no longer needed to access the contents of the file.

All of this is accomplished using the regions infrastructure
by repsenting CBFS sources and files as region_devices. Because
region_devices can be chained together forming subregions this
allows one to decouple a CBFS source from a file. This also allows
one to provide CBFS files that came from other sources for
payload and/or stage loading.

The program loading takes advantage of those very properties
by allowing multiple sources for locating a program. Because of
this we can reduce the overhead of loading programs because
it's all done in the common code paths. Only locating the
program is per source.

Change-Id: I339b84fce95f03d1dbb63a0f54a26be5eb07f7c8
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/9134


Tested-by: build bot (Jenkins)
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>