Don't search for TEENSY_LOADER_CLI if it's already defined

tmk_core/avr.mk

@@ -87,12 +87,14 @@ DEBUG_PORT = 4242
 DEBUG_HOST = localhost
 
 #============================================================================
-# Autodecct teensy loader
+# Autodetect teensy loader
+ifndef TEENSY_LOADER_CLI
     ifneq (, $(shell which teensy-loader-cli 2>/dev/null))
         TEENSY_LOADER_CLI ?= teensy-loader-cli
     else
         TEENSY_LOADER_CLI ?= teensy_loader_cli
     endif
+endif
 
 # Program the device.
 program: $(BUILD_DIR)/$(TARGET).hex $(BUILD_DIR)/$(TARGET).eep