qemu-arm/Makefile: Make the build directory reflect the board.

So multiple boards can be built at once.
Signed-off-by: Damien George <damien@micropython.org>

ports/qemu-arm/Makefile

+BOARD ?= mps2-an385
+
+# Make the build directory reflect the board.
+BUILD ?= build-$(BOARD)
+
 include ../../py/mkenv.mk
 -include mpconfigport.mk
 
@@ -11,8 +16,6 @@ MICROPY_ROM_TEXT_COMPRESSION ?= 1
 include $(TOP)/py/py.mk
 include $(TOP)/extmod/extmod.mk
 
-BOARD ?= mps2-an385
-
 ifeq ($(BOARD),netduino2)
 CFLAGS += -mthumb -mcpu=cortex-m3 -mfloat-abi=soft
 CFLAGS += -DQEMU_SOC_STM32