fix(dracut-systemd): rootfs-generator cannot write outside of generator dir

fix(dracut-systemd): rootfs-generator cannot write outside of generator dir

Although it was already documented in systemd.generator(7) that generators must
not write to locations other than those passed as arguments, since
https://github.com/systemd/systemd/commit/ca6ce62d systemd executes generators
in a mount namespace "sandbox", so now the hooks created by the rootfs-generator
are lost.

These hooks are created using the root= cmdline argument, so this patch moves
the creation of these hooks to a cmdline hook.

Fixes issue #2211
Fixes issue #2225