MIPS: Add onenand_init() to board.c and move nand_init()

This patch adds a call to onenand_init() for OneNAND support and moves
the nand_init() call to an earlier place, so that the environment can
be used from NAND and OneNAND.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>