Make the packaging script consistent with PMC's

- version number defauls to $(git describe --tag)
- fix a bug in which $pkgver wasn't expanded as quoted with '

Change-Id: Ie7d396045337aaec49d36664f05dbc73a4fb50bd
(cherry picked from commit bbb931d4741c430905fbb159aa3f1e28fcf306c8)