conf: exclude bbappends in meta-xilinx

meta-xilinx contains bbappend files for three recipes we do not provide
anywhere in our tree.  Use BBMASK to automatically remove them when using
the meta-xilinx layer.

Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>
Merge pull request #350 from joeythesaint/jjm/release-build-cleanup

conf: exclude bbappends in meta-xilinx