Instead of using /tmp for building the pkg package, use a directory
under ${OBJDIR}. This means we don't clutter /tmp (which may be a
tmpfs) with random junk, and all the build artefacts for a particular
build are in the same place.

While here, make make-pkg-package.sh fail properly if anything goes
wrong during the build, rather than just not building the package
and producing confusing errors later.