cbfs: Move result variable into the struct

Move the result variable into the struct also, so that it can be used when
BSS is not available. Add a function to read it.

Note that all functions sill use the BSS version of the data.

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