Add new macro for handling verbosity

It might be interesting to test the verbosity
level before making resource consuming stuff.
Se example below:

  if (afb_verbose_notice()) {
      char *s = compute();
      NOTICE("I got %s", s);
      free(s);
  }

Change-Id: I7be4248a86fc2a2a23820ec822b36d9fccafb203
Signed-off-by: José Bollo <jose.bollo@iot.bzh>