conf: machine: imx8m[m,p]: add support for fslc bsp
Add support for FSL Community BSP to imx8mmevk, imx8mpevk and imx8mnevk
machines and ease restriction for those machines to be built with
NXP-only BSP. FSL Community BSP allows those machines to pick up upstream
components in order to construct the image.
Bootloader provider is now selected to provide either u-boot-imx or
u-boot-fslc for mx8mm machines based on the BSP flavor selected.
WIC container is defined to accommodate ATF and option OP-TEE from NXP,
as it is required for mx8mm family to boot.
