uboot: fix target uboot defconfig warning

The warning currently reads:

  No board defconfig name specified, check your
  BR2_TARGET_UBOOT_DEFCONFIG setting.

It should read:

  No board defconfig name specified, check your
  BR2_TARGET_UBOOT_BOARD_DEFCONFIG setting.

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
(cherry picked from commit b7f095920a147c58f07b18f1b68553a1af03ae75)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/uboot/uboot.mk

@@ -293,7 +293,7 @@ endif # UBOOT_BOARD_NAME
 else ifeq ($(BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG),y)
 ifeq ($(BR2_TARGET_UBOOT_USE_DEFCONFIG),y)
 ifeq ($(call qstrip,$(BR2_TARGET_UBOOT_BOARD_DEFCONFIG)),)
-$(error No board defconfig name specified, check your BR2_TARGET_UBOOT_DEFCONFIG setting)
+$(error No board defconfig name specified, check your BR2_TARGET_UBOOT_BOARD_DEFCONFIG setting)
 endif # qstrip BR2_TARGET_UBOOT_BOARD_DEFCONFIG
 endif # BR2_TARGET_UBOOT_USE_DEFCONFIG
 ifeq ($(BR2_TARGET_UBOOT_USE_CUSTOM_CONFIG),y)