Add imx8m support:
- add a cortexa53 subtarget to imx
- move ARCH and KERNELNAME to subtargets
- account for kernel modules that are not used for cortexa53
No device-specific targets or firmware images are created yet but all
imx8m* dtbs will be built.
enabling CONFIG_TARGET_ROOTFS_INITRAMFS results in
openwrt-imx-cortexa53-imx8m-initramfs-kernel.bin which has been
successfully booted on an imx8mm-evk using the following:
u-boot=> tftpboot $fdt_addr_r image-imx8mm-evk.dtb && \
tftpboot $kernel_addr_r openwrt-imx-cortexa53-imx8m-initramfs-kernel.bin && \
booti $kernel_addr_r - $fdt_addr_r
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
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