Changed u-boot settings for sama5d3_xplained.conf

sama5d3_xplained uses the same config as sama5d3xek and there is no explicit sama5d3_xplained_nandflash_config
UBOOT_MACHINE = "sama5d3xek_nandflash_config"
Merge pull request #15 from koansoftware/patch-2

Changed u-boot settings for sama5d3_xplained.conf