doc: Mention fastboot dependency on CMDLINE

The fastboot 'boot' command only supports running a U-Boot command if
CONFIG_CMDLINE is enabled. Mention this in the documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
1 file changed