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>
diff --git a/configs/imx8mn_evk_defconfig b/configs/imx8mn_evk_defconfig
index 86601eb..e477028 100644
--- a/configs/imx8mn_evk_defconfig
+++ b/configs/imx8mn_evk_defconfig
@@ -20,6 +20,7 @@
 CONFIG_SPL=y
 CONFIG_SPL_IMX_ROMAPI_LOADADDR=0x48000000
 CONFIG_DEFAULT_DEVICE_TREE="imx8mn-evk"
+CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
 CONFIG_FIT_EXTERNAL_OFFSET=0x3000
 CONFIG_SPL_LOAD_FIT=y