nios2: fix no flash, add nand and mmc init in board.c

This patch fixes error when CONFIG_SYS_NO_FLASH. And adds
nand flash and mmc initialization, which should go before
env initialization.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>