u-boot: separate config file

Tool mkimage generates from a config file a u-boot header. A config file is
required for each u-boot header (spi, sata, uart, nand). The config file
consits of a header area and a data area. The data area is the same for all
u-boot headers.
In order not to keep the data area multiple times, the config files are
separated in a header file and a data file. And the config files are created
during the build process.

Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>
u-boot: create spi header

Create two u-boot spi header with different source addresses and a u-boot image
without header.
The u-boot image can then be loaded from two different spi addresses by Marvell
BootROM firmware.

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