configs: sama5d2_xplained: fix bootcmd/args for spi+emmc demo

For sama5d2_xplained_spiflash_defconfig, we have the demo layout
as presented on this link:
http://www.at91.com/linux4sam/bin/view/Linux4SAM/Sama5d2XplainedMainPage#SPI_eMMC_Flash_demo_Memory_map



on SPI Flash (4 Mbyte) we have Bootstrap (second level bootloader), U-boot + env

and kernel+dtb we keep on eMMC on single partition in /boot directory, formatted
with ext4.

Thus, changing the boot command to reflect this demo for the spiflash config,
and fixing up bootargs. Sama5d2_xplained does not have NAND flash, so the
bootargs were completely wrong.

Fixes: "5abc1a45": common: Move CONFIG_BOOTARGS to Kconfig
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>