imx-mkimage: allow to be used with mainline bsp for mx8m
imx-boot provides a boot container, which is required for mx8m family of
SOCs to execute a boot sequence.
Allow the imx-boot to generate boot container for Mini, Nano and Plus
SOCs by adding use-mainline-bsp to the list of compatible machines.
In order to accommodate new boot container generation model, former
SOC_TARGET variable has been renamed to IMX_BOOT_SOC_TARGET and should
be explicitly defined in the target machine recipes. This is done to
indicate that even if the mx8mm machine is finter out from
MACHINEOVERRIDES by extender filter - this recipe picks up a proper
target definition for boot container construction.
Default ATF binary image is also now based on ATF_PLATFORM, which is
explicitly defined in the machine configuration file.
Current support includes only Mini, Nano and Plus SOCs from i.MX8M
family, and additions of further machines would require the definition
of ATF_PLATFORM and IMX_BOOT_SOC_TARGET to be moved out from imx-boot
recipe to a machine configuration file.
