meta-ibm: palmetto: override variable assignments

Add overrides to variable assignments as described in "Modify Variables
to Support a Different Machine" in the yocto development tasks manual:
https://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html#best-practices-to-follow-when-creating-layers

In a nutshell this allows multiple BSP layers to be included
simultaneously in bblayers.conf, which increases the potential for meta
data sharing between layers.

(From meta-ibm rev: a7ed71c672baa533eddcc8cc713b03d2ee07c4e4)

Change-Id: Id057b3520db5952310b31f188e53263653606d54
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-ibm/meta-palmetto/recipes-phosphor/mrw/mrw-native_%.bbappend b/meta-ibm/meta-palmetto/recipes-phosphor/mrw/mrw-native_%.bbappend
index 836ef56..b5ea370 100644
--- a/meta-ibm/meta-palmetto/recipes-phosphor/mrw/mrw-native_%.bbappend
+++ b/meta-ibm/meta-palmetto/recipes-phosphor/mrw/mrw-native_%.bbappend
@@ -1,2 +1,2 @@
-FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
-SRC_URI += "file://palmetto.xml.patch.xml"
+FILESEXTRAPATHS_prepend_palmetto := "${THISDIR}/${PN}:"
+SRC_URI_append_palmetto = " file://palmetto.xml.patch.xml"