kwboot: boot kirkwood SoCs over a serial link

The kwboot program boots boards based on Marvell's Kirkwood platform
via Xmodem over their integrated UART.

Signed-off-by: Daniel Stodden <daniel.stodden@googlemail.com>
Acked-by: Luka Perkov <uboot@lukaperkov.net>
Tested-By: Holger Brunck <holger.brunck@keymile.com>
Tested-By: David Purdy <david.c.purdy@gmail.com>
Tested-by: Simon Guinot <simon.guinot@sequanux.org>