package/xenomai: add support to download from a custom git tree

package/xenomai: add support to download from a custom git tree

Soem versions are tagged, but not tarballed, so we need to be able to
retrieve them from git.

Usually, we do not have a default value for the repository, but unlike
for the kernel or the various bootloaders, the most prominent repository
for Xeniomai is upstream's official one (for the reason above, amongst
others), so we use that as a default.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
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>