bitbake.conf, collections.inc: Add COLLECTIONS mechanism w/ a sane default.

bitbake.conf, collections.inc: Add COLLECTIONS mechanism w/ a sane default.

Added code which takes a list of directories in COLLECTIONS, in priority
order (highest to lowest), and uses those to populate BBFILES,
BBFILE_COLLECTIONS, BBFILE_PATTERN_*, and BBFILE_PRIORITY_*.

The default COLLECTIONS is based on BBPATH, so you can now specify a sane
BBPATH, not bother setting up the collections variables or BBFILES, and
still likely be able to do a build.

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