meta-phosphor: linux: simplify mctp config selection

Leverage the mctp DISTRO_FEATURE to simplify the selection of the
mctp kernel config.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: If01e472b9128d1e90fd29e1e31cef284057890fe
diff --git a/meta-phosphor/recipes-kernel/linux/linux-%.bbappend b/meta-phosphor/recipes-kernel/linux/linux-%.bbappend
index db375e4..9908302 100644
--- a/meta-phosphor/recipes-kernel/linux/linux-%.bbappend
+++ b/meta-phosphor/recipes-kernel/linux/linux-%.bbappend
@@ -1,5 +1,8 @@
 FILESEXTRAPATHS:prepend := "${THISDIR}:"
 SRC_URI:append = " \
         file://systemd-bootchart.cfg \
-        ${@bb.utils.contains('DISTRO_FEATURES', 'mctp', 'file://mctp/mctp.scc file://mctp/mctp.cfg', '', d)} \
+        "
+SRC_URI:append:df-mctp = " \
+        file://mctp/mctp.scc \
+        file://mctp/mctp.cfg \
         "