make: Improve tar generation

By default it generates archive name same
as a tag name, but Pavel requested to match
old scheme and strip off "v" prefix.

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