mount: Mount /run as a tmpfs in the rootMountPoint.

In particular, this also ensures that the directory gets created on the
target system so things can be mounted into it.

Fixes #173.

src/modules/mount/mount.conf

@@ -9,6 +9,9 @@ extraMounts:
     - device: /dev
       mountPoint: /dev
       options: bind
+    - device: tmpfs
+      mountPoint: /run
+      options: tmpfs
 
 extraMountsEfi:
     - device: /sys/firmware/efi