[U-Boot] Disable features

Hi,

Is existing a way to globally disable some features such as ethernet or Nand support, instead of doing it for a specific board?

More over, I can't disable ethernet on the at91sam9261ek board by simply disable the #define CONFIG_DRIVER_DM9000 in the 'config/at91sam9261ek.h' file so it cause building errors.

Thanks for your support,

best regards,

Simon P.