yocto-compat-layer.py: Add --dependency argument

yocto-compat-layer.py: Add --dependency argument

When processing a large number of items, there are times that it would be
nice to be able to pass in a series of layers that can be used as dependencies
for the layer that is being scanned.  This avoids the significant overhead
of processing all of the layers to compatibility.

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>