AT91SAM9260EK: Normalize BOOTARGS

This patch adapts CONFIG_BOOTARGS to the chosen boot method (boot from
DataFlash or from NAND), and gives to Linux a fully specified mtdparts
variable.

Signed-off-by: Stelian Pop <stelian@popies.net>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>