The generated 'its' is passed to mkimage which expects linux arch
strings rather than the full arch (e.g. mips not mipsel).
It currently works in some cases where LINUX_KARCH == ARCH but
otherwise you get an unknown arch build error.
Signed-off-by: Ian Pozella <Ian.Pozella@imgtec.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>