imx8mn: configs: add support for distro boot commands

Supported boot device types in iMX8MN: MMC, DHCP.

Add DISTRO_DEFAULTS config option and include the distro boot command
header file to enable full support of distro boot on i.MX8M Nano EVK (both
DDR and LPDDR derivatives).

Drop previous environment, which was targeting customized boot commands and
boot order.

Signed-off-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Cc: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>