diff --git a/make/tools.mk b/make/tools.mk
index 1edac9c388..21af7582c7 100644
--- a/make/tools.mk
+++ b/make/tools.mk
@@ -15,15 +15,15 @@ ARM_SDK_DIR := $(TOOLS_DIR)/gcc-arm-none-eabi-7-2018-q2-update
 
 .PHONY: arm_sdk_install
 ifdef LINUX
-  arm_sdk_install: ARM_SDK_URL  := https://developer.arm.com/-/media/Files/downloads/gnu-rm/7-2018q2/gcc-arm-none-eabi-7-2018-q2-update-linux.tar.bz2
+  arm_sdk_install: ARM_SDK_URL  ?= https://developer.arm.com/-/media/Files/downloads/gnu-rm/7-2018q2/gcc-arm-none-eabi-7-2018-q2-update-linux.tar.bz2
 endif
 
 ifdef MACOS
-  arm_sdk_install: ARM_SDK_URL  := https://developer.arm.com/-/media/Files/downloads/gnu-rm/7-2018q2/gcc-arm-none-eabi-7-2018-q2-update-mac.tar.bz2
+  arm_sdk_install: ARM_SDK_URL  ?= https://developer.arm.com/-/media/Files/downloads/gnu-rm/7-2018q2/gcc-arm-none-eabi-7-2018-q2-update-mac.tar.bz2
 endif
 
 ifdef WINDOWS
-  arm_sdk_install: ARM_SDK_URL  := https://developer.arm.com/-/media/Files/downloads/gnu-rm/7-2018q2/gcc-arm-none-eabi-7-2018-q2-update-win32.zip
+  arm_sdk_install: ARM_SDK_URL  ?= https://developer.arm.com/-/media/Files/downloads/gnu-rm/7-2018q2/gcc-arm-none-eabi-7-2018-q2-update-win32.zip
 endif
 
 arm_sdk_install: ARM_SDK_FILE := $(notdir $(ARM_SDK_URL))