phosphor: Rename bb file collection

Typical upstream layers have a -layer suffix.  Rename
from obmc-phosphor to phosphor-layer to match this de-facto
standard.  Some existing layers that use this convention
include:

openembedded-layer
networking-layer
perl-layer
virtualization-layer

Change-Id: I5df5273b7ef4a0d8742b74a9b1ecd680a8598b04
Tested: Built a palmetto image
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>