mmc: Add read-only directory to support mboxd

The mboxd implementation assumes that there is a read-only and
read-write directory. During a write request, mboxd copies the
contents of the read-only to the read-write directory. To continue
to support mboxd, mount a read-only directory of the hostfw image
for mboxd to use. Otherwise the write command fails (can't copy
the file to itself) and mboxd exits.

Tested: Verified mboxd didn't fail and system powered on.

Change-Id: Idcb8ff0c5eee0179d5d70e60e5e01c7d0f76b880
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/vpnor/obmc-vpnor-updatesymlinks.service b/vpnor/obmc-vpnor-updatesymlinks.service
index 31ca628..7a4ed54 100644
--- a/vpnor/obmc-vpnor-updatesymlinks.service
+++ b/vpnor/obmc-vpnor-updatesymlinks.service
@@ -13,4 +13,5 @@
 ExecStart=/usr/bin/obmc-vpnor-util updatesymlinks
 
 [Install]
+WantedBy=pldmd.service
 WantedBy=mboxd.service mboxd-reload@.service