meta-skeleton: fix warnings for append operators combined with +=

(From OE-Core rev: 619273ecb6c17efd0b4d01f17688f1b211991ca2)

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-skeleton/recipes-core/busybox/busybox_%.bbappend

@@ -8,6 +8,6 @@
 # http://www.yoctoproject.org/docs/current/kernel-dev/kernel-dev.html#changing-the-configuration
 FILESEXTRAPATHS:prepend := "${THISDIR}/${PN}:"
 
-SRC_URI:append:mydistro += " \
+SRC_URI:append:mydistro = " \
 	    file://no_rfkill.cfg \
            "