boards/arduino*: Unifies PORT_<OS> settings

All boards define a default PORT_LINUX and PORT_DARWIN
arduino-atmega-common uses PORT_LINUX and PORT_DARWIN accordingly

boards/arduino-atmega-common/Makefile.include

@@ -4,9 +4,9 @@ include $(RIOTBOARD)/arduino-atmega-common/Makefile.dep
 # define port used to flash the board
 OS = $(shell uname)
 ifeq ($(OS),Linux)
-		PORT = $(LINUX_PORT)
+  PORT = $(PORT_LINUX)
 else ifeq ($(OS),Darwin)
-  PORT ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
+  PORT ?= $(PORT_DARWIN)
 else
   $(info CAUTION: No flash tool for your host system found!)
   # TODO: fix for building under windows

boards/arduino-duemilanove/Makefile.include

@@ -7,7 +7,8 @@ USEMODULE += arduino-atmega-common
 INCLUDES += -I$(RIOTBOARD)/arduino-atmega-common/include
 
 #export needed for flash rule
-export LINUX_PORT ?= /dev/ttyUSB0
+export PORT_LINUX ?= /dev/ttyUSB0
+export PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
 export PROGRAMMER_SPEED ?= 57600
 
 export FFLAGS += -p m328p

boards/arduino-mega2560/Makefile.include

@@ -7,7 +7,8 @@ USEMODULE += arduino-atmega-common
 INCLUDES += -I$(RIOTBOARD)/arduino-atmega-common/include
 
 #export needed for flash rule
-export LINUX_PORT ?= /dev/ttyACM0
+export PORT_LINUX ?= /dev/ttyACM0
+export PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
 export PROGRAMMER_SPEED ?= 115200
 
 export FFLAGS += -p m2560

boards/arduino-uno/Makefile.include

@@ -7,7 +7,8 @@ USEMODULE += arduino-atmega-common
 INCLUDES += -I$(RIOTBOARD)/arduino-atmega-common/include
 
 # export needed for flash rule
-export LINUX_PORT ?= /dev/ttyACM0
+export PORT_LINUX ?= /dev/ttyACM0
+export PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
 export PROGRAMMER_SPEED ?= 115200
 
 export FFLAGS += -p m328p

boards/arduino-zero/Makefile.include

@@ -3,8 +3,8 @@ export CPU = samd21
 export CPU_MODEL = samd21g18a
 
 # set default port depending on operating system
-PORT_LINUX ?= /dev/ttyACM0
-PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
+export PORT_LINUX ?= /dev/ttyACM0
+export PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
 
 # setup serial terminal
 include $(RIOTBOARD)/Makefile.include.serial