zephyr: force mcuboot to fit in the boot partition

Add a DTC overlay file which uses the boot partition as the code
partition.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
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;
+	};
+};