buildscript: Include build variant in firmware version label

With this change, based on a tag "20190101" two firmwares
node-20190101
layer3-20190101
can be created based on build variant.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>