mrep: download prebuilts after sync

This is needed according to the new Replicant documentation:
http://redmine.replicant.us/projects/replicant/wiki/ReplicantSources#DownloadingUpdating-the-source-from-the-repos
http://projects.goldelico.com/p/gta04-replicant/page/Replicant-4/
Merge remote-tracking branch 'goldeli/master'

Conflicts:
	scripts/mrep