improve makefeduprepo dracut call

Try not to pull useless stuff from the host system into the upgrade.img
created by makefeduprepo.
drop unneeded workaround

Plymouth gets started by other stuff, no need to mess with it here.
drop workaround for missing `swapoff`

fixed upstream as of dracut-025 (haraldh/dracut@488d55c)
fix initrd-system-upgrade-generator

The [ -d /sysroot/system-upgrade ] test fails when the link is an
absolute link; we've changed roots, so /var/tmp/system-upgrade doesn't
exist anymore.

Read the contents of the symlink and check under /sysroot instead.
Force plymouth to show details if in text mode

This removes the plymouth 'text' plugin, which should cause plymouth to
fall back to 'details' on systems where we can't do graphics.
make system-upgrade-shell.service run again after switch-root

basic.target isn't active once we get back to the initrd. sysinit is,
though.
Create /etc/system-upgrade-release in initrd

This gives us a way to identify the version of fedup that's inside a
given upgrade.img, which might be helpful.
make sure /system-upgrade-root has os-release

We remove /etc/initrd-release to turn off systemd's initrd-mode, but
dracut had /etc/os-release as a symlink to /etc/initrd-release, and
newer systemd refuses to switch-root into something without os-release.

So: symlink system-upgrade-release in place to make sure /etc/os-release
exists as expected.
copy $NEWROOT/system-upgrade link to /run

This gives us a (temporary) backup copy of the link, which can serve as a
flag for the system to run the upgrade.
Drop UPGRADEROOT, UPGRADELINK, and /run/initramfs/upgrade.conf

These paths aren't really mutable - they're part of the documented spec
at this point. If you want to change them, there's always sed.

This means we no longer need /run/initramfs/upgrade.conf to tell us
where those things are, so we can drop that, which simplifies some
things.

Also updated the docs to reflect the changes.
Use @VERSION@ in fedup-dracut.spec.in

This way we actually write the VERSION (set in Makefile) into the spec.