kselftests-bpf: add kernel BPF tests

kselftests-bpf: add kernel BPF tests

Build and package kernel self-tests used for BPF testing, program and JIT
development. This package, together with the existing 'kmod-bpf-test', was
extensively used for past upstream Linux JIT submissions [1].

Currently this includes only 'test_verifier'; building 'test_progs' will
fail due to known endian limitations with bpftool skeletons.

[1]:https://lore.kernel.org/bpf/cover.1633392335.git.Tony.Ambardar@gmail.com

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>