devicetree.bbclass: include symbols in base DT

devicetree.bbclass: include symbols in base DT

When processing overlays, the base device tree must be compiled with
symbols, otherwise attempting to apply overlays in U-Boot will fail
with:

  failed on fdt_overlay_apply(): FDT_ERR_NOTFOUND
  base fdt does did not have a /__symbols__ node
  make sure you've compiled with -@

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>