colibri_imx6/imx7/imx8x: use distroboot by default

Use distro_bootcmd as defauult bootcmd instead of legacy wrappers.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>