refpolicy: add variant that builds modular policy

refpolicy: add variant that builds modular policy

This adds a variant of refpolicy that builds the modular form of the
policy. While this requires more memory on the target device, along with
some tricks to deal with OpenWrt's volatile /var directory, it is useful
for experiementing with SELinux policy.

Signed-off-by: W. Michael Petullo <[email protected]>