Backport v2022pre <- v2023dev0: Use external and align data for the FIT image

Backport !458 (merged)

The kernel for ARM devices required the fdt to be aligned to 64 bit. Unfortunately when generate a FIT image mkimage doesn't enforce that in any way. Work around that by putting the data externally from the flat dtb and require an 64 bit (8 bytes) alignment

Signed-off-by: Sjoerd Simons sjoerd@collabora.com