package/mender: adding device_type file

In order to have the mender package working out of the box, the
/etc/mender/device_type should be present and should contain a
valid device_type value.
This patch provides a default file that can be overridden easily
from an overlay.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Acked-by: <aduskett@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/mender/device_type

@@ -0,0 +1 @@
+device_type=BUILDROOT_DEVICE

package/mender/mender.mk

@@ -53,6 +53,9 @@ define MENDER_INSTALL_CONFIG_FILES
 	$(INSTALL) -D -m 0755 package/mender/artifact_info \
 			$(TARGET_DIR)/etc/mender/artifact_info
 
+	$(INSTALL) -D -m 0755 package/mender/device_type \
+			$(TARGET_DIR)/etc/mender/device_type
+
 	ln -snf /var/run/mender $(TARGET_DIR)/var/lib/mender
 endef