upgrade-prep: don't activate autofs mounts

If we test for /proc/sys/fs/binfmt_misc/.please-unmount, systemd will
automount /proc/sys/fs/binfmt_misc. We don't really want that, so skip
any autofs entries we find.
generator: bail out correctly if $2 is empty

[ -d ] evaluates to true? Oh bash, you are the living end.
fix upgrade startup when packagedir isn't on root (#1045168)

When the generators run, we haven't mounted anything yet. So if
/system-upgrade points to something that isn't on the root or /usr
partition, the [ -d /system-upgrade ] test will fail, and the upgrade
won't start.

We don't need to check it here anyway - upgrade-prep.sh will do sanity
checks to make sure everything we need is actually mounted.