nand_spl: Fix compile problem with board_nand_init() prototype

This patch removes the now obsolete and additionally wrongly defined
board_nand_init() prototype from nand_spl/nand_boot.c.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Scott Wood <scottwood@freescale.com>

nand_spl/nand_boot.c

@@ -27,8 +27,6 @@
 
 static int nand_ecc_pos[] = CONFIG_SYS_NAND_ECCPOS;
 
-extern void board_nand_init(struct nand_chip *nand);
-
 #if (CONFIG_SYS_NAND_PAGE_SIZE <= 512)
 /*
  * NAND command for small page NAND devices (512)