image.mk: use LINUX_KARCH rather than ARCH for mkits

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>