makefile: use /etc/avrdude/avrdude.conf on linux

No need to look for arduino specific avrdude configuration
as distributions ship their own avrdude.conf file that
is compatible with arduino.

Marlin/Makefile

 AVRDUDE_PORT = $(UPLOAD_PORT)
 AVRDUDE_WRITE_FLASH = -Uflash:w:$(BUILD_DIR)/$(TARGET).hex:i
 ifeq ($(shell uname -s), Linux)
-AVRDUDE_CONF = $(ARDUINO_INSTALL_DIR)/hardware/tools/avrdude.conf
+AVRDUDE_CONF = /etc/avrdude/avrdude.conf
 else
 AVRDUDE_CONF = $(ARDUINO_INSTALL_DIR)/hardware/tools/avr/etc/avrdude.conf
 endif