Merge pull request #6195 from kYc0o/nucleo_osx_port_fix

boards/nucleo-common: fix port on OS X

boards/nucleo-common/Makefile.include

 # define the default port depending on the host OS
 PORT_LINUX ?= /dev/ttyACM0
-PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.SLAB_USBtoUART*)))
+PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
 
 # setup serial terminal
 include $(RIOTBOARD)/Makefile.include.serial