cbfs: Allow CBFS to be used in SPL

Add a new Kconfig option to enable CBFS in SPL. This can be useful when
the memory-init code is in CBFS.

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