vm-img: set layer-based template search path

To make vmware/virtualbox image recipes find template files correctly
set FILESEXTRAPATHS as "${LAYERDIR_core}/classes/vm-img:".

Signed-off-by: Anton Mikanovich <amikan@ilbers.de>

meta/classes/vm-img.bbclass

@@ -7,7 +7,7 @@
 inherit buildchroot
 inherit wic-img
 
-FILESEXTRAPATHS_prepend := "${FILE_DIRNAME}/vm-img:"
+FILESEXTRAPATHS_prepend := "${LAYERDIR_core}/classes/vm-img:"
 OVF_TEMPLATE_FILE ?= "vm-img-virtualbox.ovf.tmpl"
 SRC_URI += "file://${OVF_TEMPLATE_FILE}"