uboot-kirkwood: add uboot for pogoplug v4

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>