imx8mm: configs: add support for distro boot commands

Supported boot device types in iMX8MM: MMC, DHCP.

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

CONFIG_BOOTCOMMAND which is defined in include/configs/imx8mm_evk.h is
deleted because "run distro_bootcmd" is required to be the default boot
mode.

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.

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