meta-ibm: romulus: 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: 0ccb228f770fe8688fd926ad822244fab6926298)

Change-Id: I4f648d1c2232a6f246b63b8e46731b7ef1af13ac
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-ibm/meta-romulus/recipes-phosphor/skeleton/obmc-libobmc-intf_%.bbappend b/meta-ibm/meta-romulus/recipes-phosphor/skeleton/obmc-libobmc-intf_%.bbappend
index 72d991c..4db6f3a 100644
--- a/meta-ibm/meta-romulus/recipes-phosphor/skeleton/obmc-libobmc-intf_%.bbappend
+++ b/meta-ibm/meta-romulus/recipes-phosphor/skeleton/obmc-libobmc-intf_%.bbappend
@@ -1 +1 @@
-FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
+FILESEXTRAPATHS_prepend_romulus := "${THISDIR}/${PN}:"