Update uid/gid of build root if uid/gid is not match

This is very useful for sharing build root to other developers,
and create a full build root for using by all developer.

Change-Id: I1edd3830a499cff7ab357d17484075e46bd8319b
Add option --keep-packs to keep packages already in build root

with --keep-packs, unused packages will not be removed while resusing
buildroot. This can speed up build hundred of packages significantly.

This option also usefull to create a single buildroot to share with other
developers to build all packages.