configs/riotboard: bump u-boot to version 2015.07

Signed-off-by: Nikolay Dimitrov <picmaster@mail.bg>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

configs/riotboard_defconfig

@@ -14,7 +14,7 @@ BR2_ROOTFS_OVERLAY="board/embest/riotboard/rootfs_overlay"
 
 # bootloader
 BR2_TARGET_UBOOT=y
-BR2_TARGET_UBOOT_VERSION="2015.04"
+BR2_TARGET_UBOOT_VERSION="2015.07"
 BR2_TARGET_UBOOT_BOARDNAME="riotboard"
 BR2_TARGET_UBOOT_FORMAT_IMX=y