In order to provide media with customized releases similar to what was possible with legacy tar based distfiles, I found a view release glitches which affect non-customized releases as well.
Most notably, jail packages are missing on the offline repos as shipped,
see https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=296202
for that specific fix.
The following small changes fix less severe nits:
release: Respect BRANCH_OVERRIDE for pkgbase package creation.
release/Makefile:
Adjust clean target to clean for ${DIST} variable instead of hard dirname.
release/scripts/pkgbase-stage.lua:
While here pass -o OSVERSION together with -o ABI to pkg(1) to avoid warning and
guessing OSVERSION by pkg(1) during pkgbase package creation.