buildroot

xenomai: update download site

The new address include more recent versions of stable releases.
Therefore, newer versions than 2.6.4 can be available when required.

Signed-off-by: Erdem MEYDANLI <erdem.meydanli@punica.com.tr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>