imx: imx8mn_beacon: Fix USB booting

imx: imx8mn_beacon: Fix USB booting

The i.MX8M Nano can boot over USB using the boot ROM instead of
adding extra code to SPL to support USB drivers, etc.  However,
when booting from USB, the environment doesnt' know where to load
and causes a hang.  Fix this hang by supporting CONFIG_ENV_IS_NOWHERE=y.
It only falls back to this condition when booting from USB, so it
does not impact MMC booting.

Suggested-by: Michael Nazzareno Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>