packagegroup-core-lsb: avoid rebuilding when changing layer config

packagegroup-core-lsb: avoid rebuilding when changing layer config

It's regrettable that code in here is referring to BBFILE_COLLECTIONS,
but it is, and the result is that this packagegroup will rebuild simply
by adding or removing a layer which may be completely unrelated to
whether you've got meta-qt4 or not. Add BBFILE_COLLECTIONS to
vardepsexclude for the function to avoid this. (This was flagged up when
testing the new yocto-compat-layer script - it understandably but
undesirably reports it as a change caused by a layer).

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>