cbfs: Allow reading a file from a CBFS given its base addr

Currently we support reading a file from CBFS given the address of the end
of the ROM. Sometimes we only know the start of the CBFS. Add a function
to find a file given that.

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