meta-phosphor: prep for new override syntax

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I588025b614416c43aa2d053765ab53bacf890cb5
diff --git a/meta-phosphor/recipes-core/dropbear/dropbear_%.bbappend b/meta-phosphor/recipes-core/dropbear/dropbear_%.bbappend
index e3749ac..731f201 100644
--- a/meta-phosphor/recipes-core/dropbear/dropbear_%.bbappend
+++ b/meta-phosphor/recipes-core/dropbear/dropbear_%.bbappend
@@ -1,7 +1,7 @@
 # 0001-Only-load-dropbear-default-host-keys-if-a-key-is-not.patch
 # has been upstreamed.  This patch can be removed once we upgrade
 # to yocto 2.5 or later which will pull in the latest dropbear code.
-FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
+FILESEXTRAPATHS:prepend := "${THISDIR}/${PN}:"
 SRC_URI += "file://dropbearkey.service \
             file://localoptions.h \
             file://dropbear.default \