root.sh: Don't copy dtbs if flash-kernel is in use

Less stream lined targets like the NXP EVKs need a hand copied
dtb but the devkit doesn't so don't copy in this case.

root.sh

@@ -36,6 +36,8 @@ function setup_qemu()
 function setup_kernel()
 {
     local link
+    local fdt_source
+    fdt_source=""
 
     mkdir -p tmp/
 
@@ -55,13 +57,11 @@ function setup_kernel()
             gunzip "${basedir}/boot/Image.gz"
             ;;
         devkit*)
-            fdt_source="${basedir}"/usr/lib/linux-image-*/freescale/"${fdt_file}"
             # Reconfigure kernel package to trigger kernel hooks
 	    chroot "${basedir}" dpkg-reconfigure $(basename ${kernel_deb%%_*})
             ;;
     esac
-    cp ${fdt_source} "${basedir}/boot/dtbs/"
-    # Make sure we have the kernel available for tests
+    [ -z "${fdt_source}" ] || cp ${fdt_source} "${basedir}/boot/dtbs/"
 }