bzip SELinux policy modules in the datadir.

The 'semodule' utility can operate on compresed modules so the only
cost of this change is a slower module load time when invoking
'semodule -i' on a running system (increased CPU load due to bzip2).
That said my tests show more than 100M reduction in ext3 image size
of core-image-selinux. This last metric is a bit skewed as the image
includes two policies. Still, a ~50M reduction in the size of the
refpolicy package is significant.