wandboard: defconfig: simplify u-boot configuration

No functional change. The git hash used is actually just the 2015.07
release, so use the release tarball instead of git.  Likewise, we have a
dedicated option for u-boot.img, so use that instead of the custom format.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/wandboard_defconfig

@@ -11,11 +11,9 @@ BR2_TARGET_UBOOT=y
 BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG=y
 BR2_TARGET_UBOOT_USE_DEFCONFIG=y
 BR2_TARGET_UBOOT_BOARD_DEFCONFIG="wandboard"
-BR2_TARGET_UBOOT_CUSTOM_GIT=y
-BR2_TARGET_UBOOT_CUSTOM_REPO_URL="http://git.denx.de/u-boot.git"
-BR2_TARGET_UBOOT_CUSTOM_REPO_VERSION="33711bdd4a4dce942fb5ae85a68899a8357bdd94"
-BR2_TARGET_UBOOT_FORMAT_CUSTOM=y
-BR2_TARGET_UBOOT_FORMAT_CUSTOM_NAME="u-boot.img"
+BR2_TARGET_UBOOT_CUSTOM_VERSION=y
+BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2015.07"
+BR2_TARGET_UBOOT_FORMAT_IMG=y
 BR2_TARGET_UBOOT_SPL=y
 BR2_TARGET_UBOOT_SPL_NAME="SPL"
 BR2_TARGET_UBOOT_ENVIMAGE=y