Enable lunch command to pick up new toolchain version.

Otherwise the previously exported value will be sticky and the value
defined in the TARGET makefile is ignored.

Change-Id: Icdacb8c46034b8a24a9875b7596b31d00861512a
diff --git a/core/combo/TARGET_linux-arm.mk b/core/combo/TARGET_linux-arm.mk
index 8be89c4..bc78174 100644
--- a/core/combo/TARGET_linux-arm.mk
+++ b/core/combo/TARGET_linux-arm.mk
@@ -34,8 +34,10 @@
 TARGET_ARCH_VARIANT := armv5te
 endif
 
-ifeq ($(strip $(TARGET_GCC_VERSION)),)
-TARGET_GCC_VERSION := 4.6
+ifeq ($(strip $(TARGET_GCC_VERSION_EXP)),)
+TARGET_GCC_VERSION := 4.7
+else
+TARGET_GCC_VERSION := $(TARGET_GCC_VERSION_EXP)
 endif
 
 TARGET_ARCH_SPECIFIC_MAKEFILE := $(BUILD_COMBOS)/arch/$(TARGET_ARCH)/$(TARGET_ARCH_VARIANT).mk