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/Makefile b/Makefile
index 1e9ec42..533aff2 100644
--- a/Makefile
+++ b/Makefile
@@ -53,4 +53,7 @@
 # Needed for mbedtls config-boot.h file.
 CFLAGS += -I$(CURDIR)/boot/zephyr/include
 
+DTC_OVERLAY_FILE := $(CURDIR)/boot/zephyr/dts.overlay
+export DTC_OVERLAY_FILE
+
 include ${ZEPHYR_BASE}/Makefile.inc
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;
+	};
+};