Assuming you have the u-boot source code already (available from TI software update site), you will need to edit the following file

.../u-boot-x.x.x/include/configs/davinci.h

 

and set CFG_ENV_IS_IN_FLASH to 0.  Then you can proceed and build u-boot as you normally would

 

> make mrproper

> make davinci_config

> make