dracut: set root to satisfy internal checks

Dracut requires root to be set, even if we never use it. Set it to a
no-op value.

Closes #516

Author: zdykstra

zfsbootmenu/pre-init/zfsbootmenu-parse-commandline.sh

@@ -248,4 +248,8 @@ esac
 
 # shellcheck disable=SC2034
 rootok=1
+
+# Dracut requires root to be defined
+# shellcheck disable=SC2034
+root=zfsbootmenu
 ln -s /dev/null /dev/root 2>/dev/null