lorabox: add port to flasher parameters

This adds the port parameter to the stm32loader flash script. This was
necessary to allow flashing on macOS X where the default port is not
autodetected correctly and needs to be specified, e.g. by running:

    BOARD=lobaro-lorabox PORT=/dev/tty.SLAB_USBtoUART \
    make -C examples/lorawan flash term`

boards/lobaro-lorabox/Makefile.include

@@ -21,6 +21,6 @@ HEXFILE = $(BINFILE)
 # -u: use sector by sector erase
 # -S: swap RTS and DTR
 # -l 0x1ff: amount of sectors to erase
-FFLAGS += -e -u -S -l 0x1ff -w $(BINFILE)
+FFLAGS += -p $(PORT) -e -u -S -l 0x1ff -w $(BINFILE)
 
 TERMFLAGS +=  --set-rts 0