olimex_a20_olinuxino_lime2_defconfig: correct u-boot configuration

This defconfig currently doesn't build:
https://travis-ci.org/buildroot/buildroot-defconfig-testing/jobs/185616951

As u-boot needs dtc, so add that and change to kconfig mode like on our
other allwinner defconfigs as that is in fact what is used.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

configs/olimex_a20_olinuxino_lime2_defconfig

@@ -23,9 +23,11 @@ BR2_TARGET_ROOTFS_EXT2=y
 BR2_TARGET_ROOTFS_EXT2_4=y
 # BR2_TARGET_ROOTFS_TAR is not set
 BR2_TARGET_UBOOT=y
-BR2_TARGET_UBOOT_BOARDNAME="A20-OLinuXino-Lime2"
+BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG=y
 BR2_TARGET_UBOOT_CUSTOM_VERSION=y
 BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2016.05"
+BR2_TARGET_UBOOT_BOARD_DEFCONFIG="A20-OLinuXino-Lime2"
+BR2_TARGET_UBOOT_NEEDS_DTC=y
 BR2_TARGET_UBOOT_SPL=y
 BR2_TARGET_UBOOT_SPL_NAME="u-boot-sunxi-with-spl.bin"
 BR2_PACKAGE_HOST_GENIMAGE=y