meta-bletchley: phosphor-network: use += style for append

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I19d92019649ba990dcc33ac1bc25c4ebadc933b6
diff --git a/meta-facebook/meta-bletchley/recipes-phosphor/network/phosphor-network_%.bbappend b/meta-facebook/meta-bletchley/recipes-phosphor/network/phosphor-network_%.bbappend
index ae025bb..b91e82f 100644
--- a/meta-facebook/meta-bletchley/recipes-phosphor/network/phosphor-network_%.bbappend
+++ b/meta-facebook/meta-bletchley/recipes-phosphor/network/phosphor-network_%.bbappend
@@ -1,7 +1,9 @@
 FILESEXTRAPATHS:prepend := "${THISDIR}/${PN}:"
 
 PACKAGECONFIG:append = " sync-mac"
-SRC_URI:append = " file://config.json "
+SRC_URI += " \
+    file://config.json \
+    "
 FILES:${PN} += "${datadir}/network/*.json"
 
 do_install:append() {