package/xenomai: set 'custom' as version in case of tarball

package/xenomai: set 'custom' as version in case of tarball

When BR2_PACKAGE_XENOMAI_CUSTOM_TARBALL is selected, the xenomai package
declared an empty version, which among others means that the build directory
becomes output/build/xenomai without any version specification, and empty
version information in 'xenomai-show-info'.

Other packages that allow a custom tarball, like 'linux' and
'arm-trusted-firmware', specify 'custom' as version in this case.

Adapt the xenomai package accordingly.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>