Merge branch 'selftest-makefile-cleanup'

Andrii Nakryiko says:

====================
Fix issues with bpf_helper_defs.h usage in selftests/bpf. As part of that, fix
the way clean up is performed for libbpf and selftests/bpf. Some for Makefile
output clean ups as well.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>