BoardConfig.mk: only set KERNEL_CROSS_COMPILE when TARGET_TOOLS_PREFIX not empty

otherwise there will be compiling errors

Change-Id: I1b608e7979ba32d5ac38698d8732f0f536d8652b
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
diff --git a/.gitreview b/.gitreview
new file mode 100644
index 0000000..e565bdc
--- /dev/null
+++ b/.gitreview
@@ -0,0 +1,5 @@
+[gerrit]
+host=dev-private-review.linaro.org
+port=29418
+project=android-internal/x20/device/mediatek/mt6797
+defaultbranch=master
diff --git a/BoardConfig.mk b/BoardConfig.mk
index ae5c1cb..d99280a 100755
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -39,12 +39,14 @@
 #TARGET_TOOLCHAIN_ROOT := prebuilts/gcc/$(HOST_PREBUILT_TAG)/aarch64/aarch64-linux-android-6.2-linaro
 #TARGET_TOOLS_PREFIX := $(TARGET_TOOLCHAIN_ROOT)/bin/aarch64-linux-android-
 
+ifneq ($(TARGET_TOOLS_PREFIX),)
 ifeq ($(shell test -d $(abspath $(TOP))/$(shell dirname $(TARGET_TOOLS_PREFIX)) && echo rel),rel)
 KERNEL_CROSS_COMPILE:= $(abspath $(TOP))/$(TARGET_TOOLS_PREFIX)
 else
 # It's already absolute, no need to touch it
 KERNEL_CROSS_COMPILE:= $(TARGET_TOOLS_PREFIX)
 endif
+endif #ifneq($(TARGET_TOOLS_PREFIX),)
 
 endif