da830: add support for NAND boot mode

Add support for enabling NAND boot mode in configuration file and
add correspanding pinmux support, nand initialize function in board file.
The size required for environment variables not more than 10KB
the CONFIG_ENV_SIZE is set to 10KB from (512 << 10).

Acked-by: Nick Thompson <nick.thompson@ge.com>
Signed-off-by: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>