zephyr: restore device tree overlay

0e3fa72df4849eb42c99efb34b8617046c2e33cb removed device tree overlay
that is required to flash mcuboot to correct partition and CMake
variable that pointed to it. Restore it.

Do not restore adding "boards/${BOARD}.overlay" to the list since Zephyr
detects this file automatically now - see https://docs.zephyrproject.org/latest/application/index.html#devicetree-overlays

Signed-off-by: Sergey Koziakov <dya.eshshmai@gmail.com>
diff --git a/boot/zephyr/dts.overlay b/boot/zephyr/dts.overlay
new file mode 100644
index 0000000..74d3dfb
--- /dev/null
+++ b/boot/zephyr/dts.overlay
@@ -0,0 +1,5 @@
+/ {
+	chosen {
+		zephyr,code-partition = &boot_partition;
+	};
+};