Implement a hack to re-enable installation of the dma.conf.

The 'CONFS' entries in share/mk/bsd.confs.mk explicitly check
for the 'installconfig', but does not behave properly with the
'distribute' target.

This seems to be related to the previously-reported issues
with files within /etc in the past.

Reported by: Ben Woods
Sponsored by: The FreeBSD Foundation