scripts/package/Makefile: compare objtree with srctree instead of test KBUILD_OUTPUT

KBUILD_OUTPUT is always empty here, so it is useless to test it. But
while use O=.., objtree and srctree will be different. I compare them
instead.

Signed-off-by: Bin Wang <wbin00@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>