build: Rework @dist and @tar generation

Currently our @tar target imples that there
is a tag in form of "vX.X", if such tag is
not present in the repo we're in trouble.

So make it sane

 - if tag present then create tar from this tag
 - if tag is not present simply use git describe helper

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>