mboxd: convert virtual-pnor to PACKAGECONFIG

Change-Id: Iad65d850e224f9c869a3c14c2698bbb13faa766b
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meta-witherspoon/recipes-phosphor/mboxd/mboxd.bbappend b/meta-witherspoon/recipes-phosphor/mboxd/mboxd.bbappend
index 4139f5e..594a6ed 100644
--- a/meta-witherspoon/recipes-phosphor/mboxd/mboxd.bbappend
+++ b/meta-witherspoon/recipes-phosphor/mboxd/mboxd.bbappend
@@ -1,3 +1,3 @@
 MBOXD_FLASH_SIZE = "64M"
 
-EXTRA_OECONF := "${@oe_filter_out('enable_virtual_pnor=no', '', d)}"
+PACKAGECONFIG_append = "virtual-pnor"