uboot-kirkwood: add uboot for pogoplug v4

add an uboot able to boot a kernel in an ubi partition

This uboot also has a "recovery" feature, before
booting from flash it will try to boot a initramfs
image called "initramfs.bin" from a FAT32-formatted
USB drive connected to the USB 2.0 port.
(u-boot lacks drivers for usb 3.0 controllers)

Just rename the initramfs image and place it on
the usb drive, the uboot will load it.

In case there is no USB drive or no such file
is found, the uboot will boot from internal flash.

The whole check takes less than a second, boot times
are not impacted.

Signed-off-by: Alberto Bursi <alberto.bursi@outlook.it>

package/boot/uboot-kirkwood/Makefile

@@ -74,6 +74,11 @@ define U-Boot/pogo_e02_second_stage
   BUILD_DEVICES:=cloudengines_pogoe02
 endef
 
+define U-Boot/pogoplugv4
+  NAME:=Cloud Engines Pogoplug V4
+  BUILD_DEVICES:=cloudengines_pogoplugv4
+endef
+
 UBOOT_TARGETS := \
 	dockstar dockstar_second_stage \
 	goflexhome \
@@ -81,7 +86,8 @@ UBOOT_TARGETS := \
 	iconnect iconnect_second_stage \
 	nsa310 \
 	nsa325 \
-	pogo_e02 pogo_e02_second_stage
+	pogo_e02 pogo_e02_second_stage \
+	pogoplugv4
 
 define Build/Configure
 	$(if $(findstring _second_stage,$(BUILD_VARIANT)),