obmc-phosphor-image: avoid distro override

FOO_df-bar += "baz"

and

FOO += "${@bb.utils.contains('DISTRO_FEATURES', 'bar', 'baz', '', d)"

accomplish the same thing but the latter is the convention upstream and
also makes it easy to grep for things conditional on DISTRO_FEATURES.

(From meta-phosphor rev: 36aaf57d28c456762156cdc64fad46bdb47bda1d)

Change-Id: I6386f727acdf68370593ae1ee6dee2c0d88f700e
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-phosphor/recipes-phosphor/images/obmc-phosphor-image.bb b/meta-phosphor/recipes-phosphor/images/obmc-phosphor-image.bb
index ccd24be..d224d70 100644
--- a/meta-phosphor/recipes-phosphor/images/obmc-phosphor-image.bb
+++ b/meta-phosphor/recipes-phosphor/images/obmc-phosphor-image.bb
@@ -23,12 +23,12 @@
         obmc-software \
         obmc-system-mgmt \
         obmc-user-mgmt \
+        ${@bb.utils.contains('DISTRO_FEATURES', 'obmc-ubi-fs', 'read-only-rootfs', '', d)} \
+        ${@bb.utils.contains('DISTRO_FEATURES', 'phosphor-mmc', 'read-only-rootfs', '', d)} \
         ssh-server-dropbear \
         obmc-debug-collector \
         obmc-network-mgmt \
         obmc-settings-mgmt \
         "
-IMAGE_FEATURES_append_df-obmc-ubi-fs = " read-only-rootfs"
-IMAGE_FEATURES_append_df-phosphor-mmc = " read-only-rootfs"
 
 inherit obmc-phosphor-image