refpolicy: explicitly use .git extension for repository
Add policycoreutils-native to DEPENDS for all images inheriting from the selinux-image class.

The refpolicy package already causes this to be built and installed
in in the build sysroot. The selinux-image class depends on semodule
and setfiles though so best to have an explicit dependency.
Merge remote-tracking branch 'flihp/master'