perf tools: Make perf depend on libbpf

By adding libbpf into perf's Makefile, this patch enables perf to build
libbpf if libelf is found and neither NO_LIBELF nor NO_LIBBPF is set.

The newly introduced code is similar to how libapi and libtraceevent
are wired into Makefile.perf.

MANIFEST is also updated for 'make perf-*-src-pkg'.

Append make_no_libbpf to tools/perf/tests/make.

The 'bpf' feature check is appended into default FEATURE_TESTS and
FEATURE_DISPLAY, so perf will check the API version of bpf in
/path/to/kernel/include/uapi/linux/bpf.h. Which should not fail except
when we are trying to port this code to an old kernel.

Error messages are also updated to notify users about the lack of BPF
support in 'perf record' if libelf is missing or the BPF API check
failed.

tools/lib/bpf is added to TAG_FOLDERS to allow us to navigate libbpf
files when working on perf using tools/perf/tags.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David Ahern <dsahern@gmail.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kaixu Xia <xiakaixu@huawei.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1444826502-49291-2-git-send-email-wangnan0@huawei.com


[ Document NO_LIBBPF in Makefile.perf, noted by Jiri Olsa ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>