imx8mp: configs: add support for distro boot commands

Supported boot device types in iMX8MP: MMC.

CONFIG_CMD_PART is added for command part and CONFIG_CMD_FS_GENERIC is
for command fstype.

scriptaddr is the location in RAM where boot.scr.uimg/boot.scr will be
loaded to prior to execution. kernel_addr_r is the location in RAM where
the kernel will be loaded to. Delete unnecessary environment variables
because "run distro_bootcmd" is set to be the default boot mode.

On the iMX8MP platform I used, "mmc1" represents SD card and "mmc2"
represents eMMC.

Signed-off-by: Alice Guo <alice.guo@nxp.com>