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/init-storage

@@ -71,7 +71,7 @@ setup_etc() {
     umount "${tmpdir}"
 }
 
-datapart=$(findfs PARTLABEL=dch-data)
+datapart=$(findfs $(awk '$2=="/var"{print $1}' /etc/fstab))
 if [ -b "${datapart}" ]; then
     printf 'Found data partition: %s\n' "${datapart}"
 else