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.

(From meta-phosphor rev: 6c1407dc6835c033196be1c36daada24b9ffac18)

Change-Id: Ia963d3feb1800aa01116f522ad38ef49b7fa9a7d
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/meta-phosphor/classes/obmc-phosphor-image.bbclass b/meta-phosphor/classes/obmc-phosphor-image.bbclass
index 6e13a65..55b4261 100644
--- a/meta-phosphor/classes/obmc-phosphor-image.bbclass
+++ b/meta-phosphor/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 \