buildroot

network-manager: use a <pkg>_VERSION_MAJOR variable

Just like we do for many other GNOME-hosted tarballs, use a
<pkg>_VERSION_MAJOR variable for network-manager.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>