Merge branch 'bpf-tools-build-improvements'

Jakub Kicinski says:

====================
As promised this series addresses nits and minor issues in tools/bpf
build infra.  One GCC-7 warning which is nice to get rid of.  Dependencies
when built with OUTPUT are fixed.  make clean will now remove the
FEATURE-DUMP.* files.  PHONY target is also updated to match reality.
====================

Signed-off-by: Daniel Borkmann <[email protected]>