cbfs: Record the start address in cbfs_priv

The start address of the CBFS is used when scanning for files. It makes
sense to put this in our cbfs_priv struct and calculate it when we read
the header.

Update the code accordingly.

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