U-boot distroboot order

U-boot by default will boot to usb0, scsi0, mmc0, dhcp, and finally pxe. If an application will not use usb0 or SATA for booting, these can be disabled only allowing boot directly to emmc.

# Only boot to emmc:
env set boot_targets mmc0
env save

# Boot to USB then eMMC:
env set boot_targets usb0 mmc0
env save

# Boot only to SATA
env set boot_targets scsi0
env save