Merge branch 'autogen-bpf-helpers'

Andrii Nakryiko says:

====================
This patch set adds ability to auto-generate list of BPF helper definitions.
It relies on existing scripts/bpf_helpers_doc.py and include/uapi/linux/bpf.h
having a well-defined set of comments. bpf_helper_defs.h contains all BPF
helper signatures which stay in sync with latest bpf.h UAPI. This
auto-generated header is included from bpf_helpers.h, while all previously
hand-written BPF helper definitions are simultaneously removed in patch #3.
The end result is less manually maintained and redundant boilerplate code,
while also more consistent and well-documented set of BPF helpers. Generated
helper definitions are completely independent from a specific bpf.h on
a target system, because it doesn't use BPF_FUNC_xxx enums.

v3->v4:
- instead of libbpf's Makefile, integrate with selftest/bpf's Makefile (Alexei);

v2->v3:
- delete bpf_helper_defs.h properly (Alexei);

v1->v2:
- add bpf_helper_defs.h to .gitignore and `make clean` (Alexei).
====================

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