u-boot kirkwood: add hikirk support

An u-boot image (e.g. UART, SATA, NAND, SPI) for the 88F6282 hikirk machine
will be constructed with a header. The u-boot image with header is created
with the mkimage tool. This patch add support for 4 hikirk machines and creates
u-boot header with image. UART, SATA, NAND and SPI.

Signed-off-by: Mario Schuknecht <schuknecht@dresearch-fe.de>