This command creates the uboot.env file which resides in /mnt/boot.

A MAC address of all zeros should always be used on eFUSE enabled RCEs.

Usage: mkubootenv <path/outfile> <mac> <phy> <boot_mode> [usrvar=usrval]
 i.e.: mkubootenv /home/uboot.env 00:01:02:03:04:05 0x1 sdboot_rtems
 i.e.: mkubootenv /home/uboot.env 00:00:00:00:00:00 0x1 sdboot_linux nocm=1