buildroot

xenomai: add support for registry

Add a config option to enable the Xenomai registry, which allows accessing
internal Xenomai state.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
[Thomas:
 - add missing Config.in comment.
 - rewrap Config.in help text.
 - tweak the .mk file logic for the registry path to avoid the super
   long line.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>