scripts: Makefile: Enable creation of _symbols_ DT node for overlays

DT compiler supports -@ flag which indicates that _symbols_ node should be
automatically created during compilation.  This node is required for
using DT overlays.  The generated DTB is backward compatible, the _symbols_
node can be safely ignored if overlays are not used.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>