Arduino.mk: fix detection of avrdude.conf

When using the system path avrdude also guess where to find its config
file in the sys conf dir.

Arduino.mk

@@ -444,6 +444,9 @@ ifndef AVR_TOOLS_DIR
         ifdef SYSTEMPATH_AVR_TOOLS_DIR
             AVR_TOOLS_DIR = $(SYSTEMPATH_AVR_TOOLS_DIR)
             $(call show_config_variable,AVR_TOOLS_DIR,[AUTODETECTED],(found in $$PATH))
+            ifndef AVRDUDE_CONF
+                AVRDUDED_CONF := $(call dir_if_exists,/etc/avrdude/avrdude.conf)
+            endif
         else
             echo $(error No AVR tools directory found)
         endif # SYSTEMPATH_AVR_TOOLS_DIR

HISTORY.md

@@ -8,6 +8,7 @@ I tried to give credit whenever possible. If I have missed anyone, kindly add it
 - Fix: Do not include the Arduino header when calling generate_assembly on .cpp files. (https://github.com/Batchyx)
 - Fix: Auto-detect F_CPU on Teensy from boards.txt (https://github.com/DaWelter)
 - Tweak: Allow to disable LTO (currently crashes on Fedora 25)
+- Fix: find AVRDUDE_CONF for system avrdude
 
 ### 1.5.2 (2017-01-11)