imx8mq: configs: add support for distro boot commands

imx8mq: configs: add support for distro boot commands

Supported boot device types in iMX8MQ: MMC, DHCP.

CONFIG_DISTRO_DEFAULTS is added for enabling the DISTRO_DEFAULTS option.

CONFIG_BOOTCOMMAND which is defined in include/configs/imx8mq_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 because "run distro_bootcmd" is
set to be the default boot mode.

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