meta-openpower: phosphor-rest add distro suffix

For phosphor-rest, add df-openpower distro suffix to avoid
poisoning other machines or distros using this layer.  This is
effectively bringing this recipe up to the state of other openpower
recipes.

Change-Id: I4e3618aa68a8273bd60c09cdc3054e9f87ac3e6e
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/recipes-phosphor/interfaces/phosphor-rest.bbappend b/recipes-phosphor/interfaces/phosphor-rest.bbappend
index 72d991c..a28e3da 100644
--- a/recipes-phosphor/interfaces/phosphor-rest.bbappend
+++ b/recipes-phosphor/interfaces/phosphor-rest.bbappend
@@ -1 +1 @@
-FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
+FILESEXTRAPATHS_prepend_df-openpower := "${THISDIR}/${PN}:"