configs: update config for Atmel SAMA5D3 Xplained board

Update configuration to use the 3.18.8 kernel and update U-Boot to
2015.01. With the switch to 2015.01. use the U-Boot SPL image for the
first stage bootloader instead of the at91bootstrap. The U-Boot SPL
requires that the U-Boot image format be u-boot.img in order to load
from the SD card.

Signed-off-by: Ryan Barnett <ryanbarnett3@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>