uboot-imx

sandbox: Rename sb_cmdline_option to sandbox_cmdline_option

The new name is longer but more clearly related to sandbox.

This is in a separate patch within the same series since some comments on the
SPI series rely on it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Hung-ying Tyan <tyanh@chromium.org>