collections.inc: add collections to BBPATH, handle typos in COLLECTIONS, etc.

collections.inc: add collections to BBPATH, handle typos in COLLECTIONS, etc.

- Alters BBPATH to include the newly unpacked collections, then automatically
  re-exec's bitbake with the new BBPATH to ensure the classes/.confs/etc from
  those collections are available to bitbake.
- Handle typos in the COLLECTIONS variable by warning when glob returns nothing for a given
  item.
- Correctly handle trailing slashes in collection paths, the previous
  workaround was making the order for the collection priorities undefined.

Signed-off-by: Chris Larson <clarson@mvista.com>