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

This introduces a grep dependency, which I believe we didn't have
before, but it should be available and installed by default on all the
supported systems.

tmk_core/avr.mk

@@ -87,11 +87,13 @@ DEBUG_PORT = 4242
 DEBUG_HOST = localhost
 
 #============================================================================
-# Autodecct teensy loader
-ifneq (, $(shell which teensy-loader-cli 2>/dev/null))
-  TEENSY_LOADER_CLI ?= teensy-loader-cli
-else
-  TEENSY_LOADER_CLI ?= teensy_loader_cli
+# 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.
@@ -115,11 +117,11 @@ dfu: $(BUILD_DIR)/$(TARGET).hex sizeafter
 		echo "Error: Bootloader not found. Trying again in 5s." ;\
 		sleep 5 ;\
 	done
-ifneq (, $(findstring 0.7, $(shell $(DFU_PROGRAMMER) --version 2>&1)))
-	$(DFU_PROGRAMMER) $(MCU) erase --force
-else
-	$(DFU_PROGRAMMER) $(MCU) erase
-endif
+	if $(DFU_PROGRAMMER) --version 2>&1 | grep -q 0.7 ; then\
+		$(DFU_PROGRAMMER) $(MCU) erase --force;\
+	else\
+		$(DFU_PROGRAMMER) $(MCU) erase;\
+	fi
 	$(DFU_PROGRAMMER) $(MCU) flash $(BUILD_DIR)/$(TARGET).hex
 	$(DFU_PROGRAMMER) $(MCU) reset
 
@@ -135,11 +137,11 @@ flip-ee: $(BUILD_DIR)/$(TARGET).hex $(BUILD_DIR)/$(TARGET).eep
 	$(REMOVE) $(BUILD_DIR)/$(TARGET)eep.hex
 
 dfu-ee: $(BUILD_DIR)/$(TARGET).hex $(BUILD_DIR)/$(TARGET).eep
-ifneq (, $(findstring 0.7, $(shell dfu-programmer --version 2>&1)))
-	$(DFU_PROGRAMMER) $(MCU) flash --eeprom $(BUILD_DIR)/$(TARGET).eep
-else
-	$(DFU_PROGRAMMER) $(MCU) flash-eeprom $(BUILD_DIR)/$(TARGET).eep
-endif
+	if $(DFU_PROGRAMMER) --version 2>&1 | grep -q 0.7 ; then\
+		$(DFU_PROGRAMMER) $(MCU) flash --eeprom $(BUILD_DIR)/$(TARGET).eep;\
+	else\
+		$(DFU_PROGRAMMER) $(MCU) flash-eeprom $(BUILD_DIR)/$(TARGET).eep;\
+	fi
 	$(DFU_PROGRAMMER) $(MCU) reset
 
 # Convert hex to bin.