Fix the hardware.{txt,html} build in the release/Makefile 'reldoc'

Fix the hardware.{txt,html} build in the release/Makefile 'reldoc'
target.

The doc/share/mk/doc.commands.mk sets SVN to /usr/local/bin/svn
by default, which is not necessarily installed by the documentation
project textproc/docproj port.

Ensure SVN can be evaluated properly to include the hardware pages
by iterating through /usr/local/bin and /usr/bin and looking for
both svn and svnlite binaries, and pass the SVN variable explicitly
through env(1) in the reldoc target to avoid failures if it does not
exist.

Approved by:	re (rgrimes)
Sponsored by:	The FreeBSD Foundation