package/xenomai: move the version setting to Kconfig

package/xenomai: move the version setting to Kconfig

The xenomai version is decided in Kconfig, but the default value is
set in the .mk. This is unlike the other packages that have a version
choice (like linux, uboot...).

Move the value into Config.in.

This paves the way to using other custom sources, like from a git tree.

Note however that any existing configuration will need to be adapted to
the new settings. Adding legacy handling is non-trivial, so manual
intervention will be required.

Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
cc: Romain Naour <romain.naour@gmail.com>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>