arm: do not copy magic 4 bytes of appended DTB in zImage

If the passed zImage happens to have a DTB appended, then the magic 4 bytes
of the DTB are copied together with the kernel image. This leads to
failed kexec boots because the decompressor finds the aforementioned
DTB magic and falsely tries to replace the DTB passed in the register r2
with the non-existent appended one.

Signed-off-by: Alexander Egorenkov <egorenar-dev@posteo.net>
Signed-off-by: Simon Horman <horms@verge.net.au>