boards/msba2: Do not set PORT.

Boards should not set PORT and should not have code conditional on
PORT as that causes PORT to be evaluated and the build to fail even
if this varible is not needed.

Exporting has the same effect.

This fixes the MSBA2 board by declaring PORT_LINUX and PORT_DARWIN
instead.

boards/common/msba2/Makefile.include

@@ -4,7 +4,11 @@ export CPU = lpc2387
 FLASHER = lpc2k_pgm
 
 # configure serial interface
-export TERMFLAGS += -tg -p "$(PORT)"
+PORT_LINUX ?= /dev/ttyUSB0
+# This does not make a lot of sense, but it has the same value as the previous code
+PORT_DARWIN ?= /dev/tty.usbserial-ARM
+
+TERMFLAGS += -tg -p "$(PORT)"
 include $(RIOTMAKE)/tools/serial.inc.mk
 
 export CFLAGS_CPU   = -mcpu=arm7tdmi-s
@@ -21,10 +25,7 @@ export LINKFLAGS += -Wl,--gc-sections
 # use the nano-specs of Newlib when available
 USEMODULE += newlib_nano
 
-ifeq ($(PORT),)
-  export PORT = /dev/ttyUSB0
-endif
-export FFLAGS = $(PORT) $(HEXFILE)
+FFLAGS = $(PORT) $(HEXFILE)
 
 INCLUDES += -I$(RIOTBOARD)/common/msba2/include