fs: Add a Coreboot Filesystem (CBFS) driver and commands

This change adds CBFS support and some commands to use it to u-boot. These
commands are:

cbfsinit - Initialize CBFS support and pull all metadata into RAM. The end of
the ROM is an optional parameter which defaults to the standard 0xffffffff and
can be used to support multiple CBFSes in a system. The last one set up with
cbfsinit is the one that will be used.

cbfsinfo - Print information from the CBFS header.

cbfsls - Print out the size, type, and name of all the files in the current
CBFS. Recognized types are translated into symbolic names.

cbfsload - Load a file from CBFS into memory. Like the similar command for fat
filesystems, you can optionally provide a maximum size.

Support for CBFS is compiled in when the CONFIG_CMD_CBFS option is specified.

The CBFS driver can also be used programmatically from within u-boot.

If u-boot needs something out of CBFS very early before the heap is
configured, it won't be able to use the normal CBFS support which caches some
information in memory it allocates from the heap. The
cbfs_file_find_uncached function searches a CBFS instance without touching
the heap.

Signed-off-by: Gabe Black <gabeblack@google.com>
Signed-off-by: Stefan Reinauer <reinauer@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>