payloads/LinuxBoot: Add ARM64 support

Add support for ARCH=ARM64 and introduce CROSS_COMPILE for all architectures.

* Always compress kernel Image using xz
* Create FIT uImage containing the kernel, initramfs and DTB
* Add ARM64 defconfig for all SoCs

Change-Id: I9a0cc248283432fb2384956ca55e687d4127398c
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/25152
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
7 files changed