board: phytec: common: k3: Add missing boot source to env

We set the boot source as environment variable 'boot'.
Also include 'uart' and 'usbdfu' as possible boot sources.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Reviewed-by: Wadim Egorov <w.egorov@phytec.de>
1 file changed