Read /etc/fstab to determine data partition

Rather than hard-code the GPT partition label into the `init-storage`
and `factory-reset` scripts, these now determine the block device by
reading `/etc/fstab` and using the device specified for `/var`.

overlay/usr/libexec/factory-reset

@@ -1,7 +1,7 @@
 #!/bin/sh
 # vim: set sw=4 ts=4 sts=4 et :
 
-DATAPART=PARTLABEL=dch-data
+DATAPART=$(awk '$2=="/var"{print $1}' /etc/fstab)
 
 seq 1 30