cbfs: Rename camel-case variables

Rename some camel-case variables to match U-Boot style.

Camel case is not generally allowed in U-Boot. Rename this variable to fit
in with the style.

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