Rebuild plugins and core only by flushing it completely.  Ports
can stay.  Builds will pick up where they left of in any case.

This bug exists since December 2017 where we had to side-step
fetch-recursive because FLAVOUR git in the way...

This has been obsoleted in opnsense-update and shall be removed
for our 19.1 builds.  Hooray for progress.

Reported via IRC.

Build steps for packages predate the .pkg-err file so we had no
way of knowning about the integrity of a build.  Now we can
directly check the status of the build.

The side effect is that "make plugins" and "make core" will run
"make ports" if that failed and fail there.  While there's a
little overhead in automated scripts this is more consistent with
build flow as we do not start higher targets that depend on broken
dependencies that look correct.  This was reported a few times
over the years.

Also see: https://github.com/opnsense/ports/commit/d188643649f7

This reverts commit 634a0866.
Build error, try again in 19.1.

/var MFS kills all of the files and it's extremely difficult to
construct a safe health check around the fact that they can be
missing.