buildroot

xenomai: group skin selection in separate menu and cleanup

Clarify the Xenomai config options by grouping the skin-related options in a
separate menu.

Additionally:
- update proper capitalization of skin names
- sort entries
- replace 'skin library' by 'skin' to match the terminology in Xenomai
  sources

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>