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/mmc/obmc-flash-bios-patch.service b/mmc/obmc-flash-bios-patch.service
index 226c019..2b3a941 100644
--- a/mmc/obmc-flash-bios-patch.service
+++ b/mmc/obmc-flash-bios-patch.service
@@ -1,5 +1,6 @@
 [Unit]
 Description=Create patch directory for Host FW
+Before=mboxd.service
 Before=pldmd.service
 Wants=usr-local.mount
 After=usr-local.mount