linux: add option to enable support for Device Tree overlays

Add an option to compile device trees in Linux with symbol generation
such that device tree overlays can be loaded on the target system

Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>
[Arnout: remove "default n" and move setting of LINUX_MAKE_ENV to the
 place where the rest is set.]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>