pkgbootstrap: Gate building from ports

[cperciva]

Summary

In release builds we want to build pkg from the ports tree for reasons
mentioned in a previous commit; but non-release pkgbase builds may be
just fine with installing pkg from pkg.freebsd.org.

Gate it behind BOOTSTRAP_PKG_FROM_PORTS rather than building form ports
any time we have a ports tree available.

While I'm here, make the pkgbase-repo target in release/Makefile
create the temporary directories we need for building.

MFC after: 6 hours
Sponsored by: https://www.patreon.com/cperciva
Fixes: 2c06b824436d ("pkgbootstrap: Build pkg if PORTSDIR exists")

[ivy]

i'm okay with this on the basis that it turns the pkg problem into a release thing and not something that affects all developers. we definitely need a better way to do this, but i think everyone is aware of that at this point :-)

release/Makefile

198

i really don't like the hardcoded paths here, i'd like to see this fixed to use mkdtemp(1), although i won't insist on that for this change.

[cperciva]

release/Makefile

198

Yeah, using a temporary directory, or having it configured elsewhere, would be better; but right now I'm in "get this thing building" mode rather than "do it cleanly" mode. :-/