stages: add debug shell

This gives shell access into the image on a given tty. Useful for
testing and debugging, while minimally affecting the image.

Note that this must never be used in production, as it allows root
access without a password.

For instance this could be used to verify that an image was fully
booted:

```
[teg@teg-x270 osbuild]$ qemu-kvm -m 1024 -nographic -serial mon:stdio -snapshot  base.qcow2
sh-5.0# systemctl is-system-running --wait
running
```

Signed-off-by: Tom Gundersen <teg@jklm.no>

samples/base-qcow2.json

@@ -50,7 +50,13 @@
       "name": "org.osbuild.grub2",
       "options": {
         "root_fs_uuid": "76a22bf4-f153-4541-b6c7-0332c0dfaeac",
-        "kernel_opts": "ro biosdevname=0 net.ifnames=0 console=ttyS0,115200"
+        "kernel_opts": "ro biosdevname=0 net.ifnames=0"
+      }
+    },
+    {
+      "name": "org.osbuild.debug-shell",
+      "options": {
+        "tty": "/dev/ttyS0"
       }
     },
     {