selftests/bpf: Copy extras in out-of-srctree builds

Building selftests in a separate directory like this:

    make O="$BUILD" -C tools/testing/selftests/bpf

and then running:

    cd "$BUILD" && ./test_progs -t btf

causes all the non-flavored btf_dump_test_case_*.c tests to fail,
because these files are not copied to where test_progs expects to find
them.

Fix by not skipping EXT-COPY when the original $(OUTPUT) is not empty
(lib.mk sets it to $(shell pwd) in that case) and using rsync instead
of cp: cp fails because e.g. urandom_read is being copied into itself,
and rsync simply skips such cases. rsync is already used by kselftests
and therefore is not a new dependency.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210224111445.102342-1-iii@linux.ibm.com