Allow KERNEL_CONFIG to be overridden

With this change we can set the KERNEL_CONFIG as part of
build configuration. This way we can test different
config fragments without manual changes.

Change-Id: I077d680a018b380068a89f2c612bd0ef98b7ba85
Signed-off-by: Vishal Bhoj <vishal.bhoj@linaro.org>
diff --git a/BoardConfig.mk b/BoardConfig.mk
index 35f254f..5be06e2 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -43,7 +43,7 @@
 # Juice
 #
 KERNEL_TOOLS_PREFIX := aarch64-linux-android-
-KERNEL_CONFIG := linaro/configs/linaro-base.conf \
+KERNEL_CONFIG ?= linaro/configs/linaro-base.conf \
                  linaro/configs/android.conf \
                  linaro/configs/vexpress64.conf \
                  linaro/configs/vexpress-tuning.conf \
@@ -57,7 +57,7 @@
 # IKS
 #
 ifeq ($(KERNEL_CONFIG),)
-KERNEL_CONFIG := linaro/configs/linaro-base.conf \
+KERNEL_CONFIG ?= linaro/configs/linaro-base.conf \
                  linaro/configs/android.conf \
                  linaro/configs/big-LITTLE-MP.conf \
                  linaro/configs/vexpress.conf \
@@ -82,7 +82,7 @@
 #
 # MP
 #
-KERNEL_CONFIG := linaro/configs/linaro-base.conf \
+KERNEL_CONFIG ?= linaro/configs/linaro-base.conf \
                  linaro/configs/android.conf \
                  linaro/configs/big-LITTLE-MP.conf \
                  linaro/configs/vexpress.conf \