obmc-phosphor-image: Enable read-only-rootfs for mmc distro

Set the rootfs to read-only if the mmc distro feature is enabled.
Similar to ubi, the ext4 filesystem option would have a read-only root
fs and only allow certain directories like /var and /etc to be writable.

Change-Id: I5153a451774c3a3d6d660309907f45572f70f01f
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/classes/obmc-phosphor-image.bbclass b/classes/obmc-phosphor-image.bbclass
index 6e13a65..55b4261 100644
--- a/classes/obmc-phosphor-image.bbclass
+++ b/classes/obmc-phosphor-image.bbclass
@@ -81,6 +81,7 @@
         "
 
 IMAGE_FEATURES_append_df-obmc-ubi-fs = " read-only-rootfs"
+IMAGE_FEATURES_append_df-phosphor-mmc = " read-only-rootfs"
 
 CORE_IMAGE_EXTRA_INSTALL_append = " bash \
         packagegroup-obmc-apps-extras \