conf: machine: imx8mq-evk: add mainline bsp support
Recent versions of upstream U-Boot and Kernel do provide support for
i.MX8MQ derivative, and it can be built using Mainline BSP.
Convert machine definition to include all parts, that are required to
build Mainline BSP:
- Drop hardcoded BSP flavor setting in machine configuration
- Add boot container machine override
- Split machine features
- Define ATF load address
- Add dependency to firmware package
- Provide extlinux configuration to utilize distro boot feature
