commands: usbgadget: provide fallback to global variables
usbgadget autostarter is providing now a global variable to configure fastboot.
We can use is as fallback for the cmd_usbgadget as well.
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>