Force the creation of udev rules link

Ensure that rc.local doesn't throw an error if the link to our udev
rules should somehow already exist. Also create it during "make
sys-usb", so that "udevadm control --reload" has something to work
with and sys-usb doesn't have to be rebooted for auto attachment to
start working.

dom0/sys-usb.sls

@@ -23,12 +23,12 @@ sd-rc-local-udev-rules:
     - marker_end: "### END securedrop-workstation ###"
     - content: |
         # Add udev rules for export devices
-        ln -s /rw/config/sd/etc/udev/rules.d/99-sd-export-usb.rules /etc/udev/rules.d/
+        ln -sf /rw/config/sd/etc/udev/rules.d/99-sd-export-usb.rules /etc/udev/rules.d/
         udevadm control --reload
     - require:
       - file: sd-udev-rules
   cmd.run:
-    - name: udevadm control --reload
+    - name: ln -sf /rw/config/sd/etc/udev/rules.d/99-sd-export-usb.rules /etc/udev/rules.d/ && udevadm control --reload
     - require:
       - file: sd-rc-local-udev-rules