Added default extra_flags parameter to allow build on all boards

hardware/arduino/avr/platform.txt

@@ -20,6 +20,8 @@ compiler.elf2hex.flags=-O ihex -R .eeprom
 compiler.elf2hex.cmd=avr-objcopy
 compiler.ldflags=
 compiler.size.cmd=avr-size
+# this can be overriden in boards.txt
+build.extra_flags=
 
 # AVR compile patterns
 # --------------------