Mount the root filesystem read-write by default

The root filesystem was ro, with special sections marked as rw for the
old upgrade system. Now that we have ostree that's really quite
redundant.

apertis-image-uefi.yaml

@@ -47,7 +47,7 @@ actions:
       - mountpoint: /
         partition: system
         flags: [ boot ]
-        options: [ ro ]
+        options: [ rw ]
       - mountpoint: /boot/efi
         partition: EFI
       - mountpoint: /home