Merge branch 'bpf-bpftool-build-improvements'

Merge branch 'bpf-bpftool-build-improvements'

Quentin Monnet says:

====================
This set attempts to make it easier to build bpftool, in particular when
passing a specific output directory. This is a follow-up to the
conversation held last month by Lorenz, Ilya and Jakub [0].

The first patch is a minor fix to bpftool's Makefile, regarding the
retrieval of kernel version (which currently prints a non-relevant make
warning on some invocations).

Second patch improves the Makefile commands to support more "make"
invocations, or to fix building with custom output directory. On Jakub's
suggestion, a script is also added to BPF selftests in order to keep track
of the supported build variants.

Building bpftool with "make tools/bpf" from the top of the repository
generates files in "libbpf/" and "feature/" directories under tools/bpf/
and tools/bpf/bpftool/. The third patch ensures such directories are taken
care of on "make clean", and add them to the relevant .gitignore files.

At last, fourth patch is a sligthly modified version of Ilya's fix
regarding libbpf.a appearing twice on the linking command for bpftool.

[0] https://lore.kernel.org/bpf/CACAyw9-CWRHVH3TJ=Tke2x8YiLsH47sLCijdp=V+5M836R9aAA@mail.gmail.com/

v2:
- Return error from check script if one of the make invocations returns
  non-zero (even if binary is successfully produced).
- Run "make clean" from bpf/ and not only bpf/bpftool/ in that same script,
  when relevant.
- Add a patch to clean up generated "feature/" and "libbpf/" directories.
====================

Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Lorenz Bauer <lmb@cloudflare.com>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>