Fix inconsistent usage of append

Author: scthunderbolt

conf/layer.conf

@@ -18,4 +18,4 @@ require ${@bb.utils.contains('DISTRO_FEATURES', 'em_extender','conf/include/rdk-
 LAYERSERIES_COMPAT_cmf-bananapi = " kirkstone"
 
 MIDDLEWARE_ARCH = "${MACHINE}"
-PACKAGE_EXTRA_ARCHS:append = " ${MIDDLEWARE_ARCH}"
+PACKAGE_EXTRA_ARCHS_append = " ${MIDDLEWARE_ARCH}"