tools: bpf: consistent make bpf_install

Currently, make bpf_install in tools/ does not respect DESTDIR. Moreover, it
installs to /usr/bin/ unconditionally.

Let it respect DESTDIR and allow prefix to be specified. Also, to be more
consistent with bpftool and with the usual customs, default the prefix to
/usr/local instead of /usr.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>