add system-upgrade-generator

Rather than depending on 'systemd.unit=system-upgrade.target', look for
the files that indicate that the system is ready for upgrade:

1. /system-upgrade (symlink to directory)
2. /system-upgrade-root (directory)
3. /run/system-upgrade (symlink) *or* /run/initramfs/upgrade.conf (file)

If those three conditions are met, the system is prepared for upgrade,
so (for this boot only) make 'system-upgrade.target' the default taget.

Note that /run/initramfs/upgrade.conf isn't part of the SystemUpgrade
spec, but it's retained here for compatibility with existing Fedora
releases.
drop extra boot args (#964303)

We have a systemd generator that starts the upgrade, so we don't need to
put 'systemd.unit=upgrade.unit' in the boot args to make that happen.

As for SELinux stuff, eparis says that if the upgrade breaks because of
incompatibilities between the two policies, that's a bug in the policy
and I should make the SELinux guys fix it. Works for me!

This will fix problems where people run fedup, then install a new
kernel and end up with 'systemd.unit=upgrade.unit' in their boot args.

github: fixes #24
drop /run/initramfs/upgrade.conf

Nothing's going to change these paths; let's just simplify things and
not mess around with that file. If we're in upgrade-switch-root.target
and there's a mountpoint at /system-upgrade-root, we're good to go.
update .gitignore
systemd: Makefile improvements

Clean up systemd/ Makefile a bit and do autoconf-esque replacement of
@UTILDIR@ and @LIBEXECDIR@ to generate upgrade-prep.service and
system-upgrade-generator at build time.
upgrade-prep.sh: clean up backward-compat junk a bit

Eventually we'll be able to remove that code altogether; for now just
make sure it's conditional so we don't interfere with newer upgrade.img