examples/bpf: move from test directory

examples/bpf: move from test directory

The bpf folder didn't actual contain a test application, but instead
basic examples of BPF code for use with testpmd. Therefore we can
move it to the `examples` folder. Being different, it also needs
a README with it, explaining what it is and how to use it. References
to the code from the testpmd docs are suitably updated.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>