devkit8000: Implement and activate direct OS boot

- Implements spl_start_uboot() for devkit8000
- Add configs to activate direct OS boot from SPL

Signed-off-by: Simon Schwarz <simonschwarzcor@gmail.com>
CC: Tom Rini <tom.rini@gmail.com>
CC: Stefano Babic <sbabic@denx.de>
CC: Wolfgang Denk <wd@denx.de>