cookerdata: Allow bblayers.conf to be found using BBPATH

cookerdata: Allow bblayers.conf to be found using BBPATH

It should be possible to run a build anywhere on the filesystem and have
bitbake find the correct build directory if its set somehow. The BBPATH
variable makes perfect sense for this usage. Therefore use any available
value of BBPATH to search for conf/bblayers.conf before walking the parent
directory structure.

This restores the option of being able to run bitbake from anywhere if
the user has set things up to operate in that environment.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>