mac80211: support radiotap vendor namespace RX data

In some cases, in particular for experimentation, it
can be useful to be able to add vendor namespace data
to received frames in addition to the normal radiotap
data.

Allow doing this through mac80211 by adding fields to
the RX status descriptor that describe the data while
the data itself is prepended to the frame.

Also add some example code to hwsim, but don't enable
it because it doesn't use a proper OUI identifier.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>