buildroot

xenomai: native skin is now called alchemy

Since the introduction of Xenomai 3, the native skin is dubbed Alchemy.
Update the config menu (but leave the symbol name to avoid hurting existing
users needlessly) and fix the unselected-skin cleanup code.

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