[coreboot] New CMOS option sata_mode

[Josef Kellermann]

Attached patch implements a new CMOS option 'sata_mode'.
A new Kconfig option 'SATA_MODE' with default 'ide' is added to 
amd/sb600/Kconfig.
The handling of the CMOS option is placed in amd/sb600/sata.c and 
overrides the hardcoded 'ide' mode
in the following way:
if get_option(., "sata_mode") returns 0 use CMOS option
in case of -2 grab the Kconfig option CONFIG_SATA_MODE if exist
otherwise use the hardcoded mode.

Signed-off-by: Josef Kellermann