vexpress: IKS: all overriding kernel config

Change-Id: Icffd4e4dfb0f48272d018ad9c6dee92828c11bcd
Signed-off-by: Vishal Bhoj <vishal.bhoj@linaro.org>
diff --git a/BoardConfig.mk b/BoardConfig.mk
index 9da53bf..09af805 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -42,7 +42,9 @@
 CUSTOM_BOOTLOADER_MAKEFILE := boot-wrapper/bootwrapper.mk
 endif
 else
+ifeq ($(KERNEL_CONFIG),)
 KERNEL_CONFIG := vexpress_bL_defconfig
+endif
 TARGET_USE_UBOOT := true
 UBOOT_CONFIG := vexpress_ca5x2
 DEVICE_TREES := vexpress-v2p-ca15-tc2:v2p-ca15-tc2.dtb