Remove board_init_f function from nand_boot.c

Remove the board_init_f function from nand_spl/nand_boot.c. This
 function is to be defined by all boards using the nand_spl
 functionality in their individual board directory.

 Currently this function was being used by the smdk6400 board. Added
 the board specific function definition.

Signed-off-by: Sughosh Ganu <urwithsughosh@gmail.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>