introduce a new logic to handle release numbers when a significant change is made...

introduce a new logic to handle release numbers when a significant change is made on a package:
 - release packages get a "patch level" added to their release number, this last one being incremented on every change ;
 - trunk packages should always get their release number incremented.

git-svn-id: svn://svn.openwrt.org/openwrt/packages@15321 3c298f89-4303-0410-b956-a3cf2f4a3e73