board: imx8mm: add boot.cmd for distro boot on iMX8MM

Distro Boot requires a U-Boot-specific script named boot.scr or
boot.scr.uimg which contains boot commands to boot the system. The
boot.cmd is such a file. Use mkimage to generate boot.scr or
boot.scr.uimg from boot.cmd, and the command is:
mkimage -A arm -O linux -T script -C none -a 0 -e 0 -n "Distro Boot Script" -d boot.cmd boot.scr.uimg

The boot.cmd file is an example script and can be modified based on
needs. bootargs is set in this script and root uses the default value "
/dev/mmcblk1p2 rootwait rw" which can be changed by overriding mmcroot.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Alice Guo <alice.guo@nxp.com>