cbfs: Add functions to support multiple CBFSs

Sometimes an image has multiple CBFS. The current CBFS API is limited to
handling only one at time. Also it keeps track of the CBFS internally in
BSS, which does not work before relocation, for example.

Add a few new functions to overcome these limitations.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>