ath79: add d-link dir-825-c1 and dir-835-a1

This commit ports both dir-825-c1 and dir-835-a1 from ar71xx to ath79.
They're pretty much identical, except dir-835-a1 has less LEDs.

The routers come with 128 MByte of RAM and 16 MBytes of flash and sport
2.4GHz and 5.0GHz wireless. Both routers have entries already in
OpenWrt's TOH. Please check there for more information on these
antiquities.

https://openwrt.org/toh/hwdata/d-link/d-link_dir-825_c1
https://openwrt.org/toh/hwdata/d-link/d-link_dir-835_a1

Installation:

1. Connect to the web interface of the vendor firmware (usually
   listening on 192.168.0.1).
2. Go to "Tools", then "Firmware".
3. In the "Firmware Upgrade" box click "Browse".
4. Select the OpenWrt factory image for your router.
5. Click "Upload", confirm the popups if you agree to flash the file you
   selected.
6. Wait for firmware upgrade to complete. It takes about 5 minutes.

Run-tested on dir-825-c1. dir-835-a1 should work as well, but I don't
have this router so I can't confirm.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com> [trivial changes]

include/image-commands.mk

@@ -56,6 +56,18 @@ define Build/cameo-factory
 	mv $@.new $@
 endef
 
+# blow up binary to given size and put a given string at its end:
+# cameo-factory <size[k]> <string>
+define Build/cameo-factory
+	factory_stamp=$(word 2,$(1)); \
+	((reduced_size=$(subst k,*1024,$(word 1,$(1)))-$${#factory_stamp})); \
+	( \
+		dd if=$@ bs=$$reduced_size conv=sync; \
+		echo -n $$factory_stamp; \
+	) > $@.new && \
+	mv $@.new $@
+endef
+
 define Build/eva-image
 	$(STAGING_DIR_HOST)/bin/lzma2eva $(KERNEL_LOADADDR) $(KERNEL_LOADADDR) $@ $@.new
 	mv $@.new $@