diff --git a/make/tools.mk b/make/tools.mk
index 7a889ec3a7..c3f251404d 100644
--- a/make/tools.mk
+++ b/make/tools.mk
@@ -14,7 +14,7 @@
 ##############################
 
 # Set up ARM (STM32) SDK
-ARM_SDK_DIR := $(TOOLS_DIR)/gcc-arm-none-eabi-6-2017-q1-update
+ARM_SDK_DIR ?= $(TOOLS_DIR)/gcc-arm-none-eabi-6-2017-q1-update
 # Checked below, Should match the output of $(shell arm-none-eabi-gcc -dumpversion)
 GCC_REQUIRED_VERSION ?= 6.3.1
 
@@ -286,16 +286,17 @@ zip_clean:
 #
 ##############################
 
-GCC_VERSION=$(shell arm-none-eabi-gcc -dumpversion)
 ifeq ($(shell [ -d "$(ARM_SDK_DIR)" ] && echo "exists"), exists)
   ARM_SDK_PREFIX := $(ARM_SDK_DIR)/bin/arm-none-eabi-
 else ifeq (,$(findstring _install,$(MAKECMDGOALS)))
+  GCC_VERSION = $(shell arm-none-eabi-gcc -dumpversion)
   ifeq ($(GCC_VERSION),)
     $(error **ERROR** arm-none-eabi-gcc not in the PATH. Run 'make arm_sdk_install' to install automatically in the tools folder of this repo)
   else ifneq ($(GCC_VERSION), $(GCC_REQUIRED_VERSION))
     $(error **ERROR** your arm-none-eabi-gcc is '$(GCC_VERSION)', but '$(GCC_REQUIRED_VERSION)' is expected. Override with 'GCC_REQUIRED_VERSION' in make/local.mk or run 'make arm_sdk_install' to install the right version automatically in the tools folder of this repo)
   endif
-  # not installed, hope it's in the path...
+
+  # ARM tookchain is in the path, and the version is what's required.
   ARM_SDK_PREFIX ?= arm-none-eabi-
 endif