ipq40xx: add cpximg to flash Compex WPJ428 via bootloader

Generate a cpximg that is compatible with the cpximg loader in Compex' u-boot.
The cpximg loader can be started either by holding the reset button during power
up or by entering the u-boot prompt and entering 'cpximg'.
Once it's running, a TFTP-server under 192.168.1.1 will accept an image
appropriate for the board revision that is etched on the board (e.g. 6A04).

The image can be pushed using tftp:
  tftp -v -m binary 192.168.1.1 -c put openwrt-ipq40xx-generic-compex_wpj428-squashfs-cpximg-6a04.bin

cpximg files can also be used with the sysupgrade utility in stock images.
(add SSH key in luci for root access)

In mkmylofw_32m, the calculation of the "partition size" has been preferred
to just padding the partition as this will result in less block transfers
during flashing (while the additional complexity is bearable).

Signed-off-by: Leon M. George <leon@georgemail.eu>
Co-developed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

target/linux/ipq40xx/image/Makefile

@@ -53,6 +53,22 @@ define Device/DniImage
 endef
 DEVICE_VARS += NETGEAR_BOARD_ID NETGEAR_HW_ID
 
+define Build/mkmylofw_32m
+	$(eval device_id=$(word 1,$(1)))
+	$(eval revision=$(word 2,$(1)))
+
+	let \
+		size="$$(stat -c%s $@)" \
+		pad="$(subst k,* 1024,$(BLOCKSIZE))" \
+		pad="(pad - (size % pad)) % pad" \
+		newsize='size + pad'; \
+		$(STAGING_DIR_HOST)/bin/mkmylofw \
+		-B WPE72 -i 0x11f6:$(device_id):0x11f6:$(device_id) -r $(revision) \
+		-s 0x2000000 -p0x180000:$$newsize:al:0x80208000:"OpenWrt":$@ \
+		$@.new
+	@mv $@.new $@
+endef
+
 define Build/SenaoFW
 	-$(STAGING_DIR_HOST)/bin/mksenaofw \
 		-n $(BOARD_NAME) -r $(VENDOR_ID) -p $(1) \
@@ -243,8 +259,9 @@ define Device/compex_wpj428
 	BLOCKSIZE := 64k
 	IMAGE_SIZE := 31232k
 	KERNEL_SIZE := 4096k
-	IMAGES = sysupgrade.bin
+	IMAGES = sysupgrade.bin cpximg-6a04.bin
 	IMAGE/sysupgrade.bin := append-kernel | append-rootfs | pad-rootfs | append-metadata
+	IMAGE/cpximg-6a04.bin := append-kernel | append-rootfs | pad-rootfs | mkmylofw_32m 0x8A2 3
 	DEVICE_PACKAGES := kmod-gpio-beeper
 endef
 TARGET_DEVICES += compex_wpj428