etc: Set DISTBASE during distribute for sub-makes

This is normally done by bsd.subdir.mk in its default distribute target,
but since etc overrides it we don't get that behaviour. Currently it's
not needed, but share/man's makedb (recursed into by etc's afterinstall)
will need it in the following commit. Technically this is only needed
for install, but do this for distribution too for completeness and
consistency.

Reviewed by: brooks, emaste
Differential Revision: https://reviews.freebsd.org/D43675