configs/riotboard: Decouple u-boot version from the default value

Use custom version for U-Boot, instead of using the default value as defined
in BR2_TARGET_UBOOT_VERSION.

Signed-off-by: Nikolay Dimitrov <picmaster@mail.bg>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/riotboard_defconfig

@@ -14,7 +14,8 @@ BR2_ROOTFS_OVERLAY="board/embest/riotboard/rootfs_overlay"
 
 # bootloader
 BR2_TARGET_UBOOT=y
-BR2_TARGET_UBOOT_VERSION="2015.07"
+BR2_TARGET_UBOOT_CUSTOM_VERSION=y
+BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2015.07"
 BR2_TARGET_UBOOT_BOARDNAME="riotboard"
 BR2_TARGET_UBOOT_FORMAT_IMX=y