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"

Signed-off-by: Marco Cavallini m.cavallini@koansoftware.com