add forgotten flag for NETDUINO2PLUS

Author: martinayotte

STM32F4/boards.txt

@@ -79,7 +79,7 @@ netduino2plus.upload.dfuse_addr=0x8000000
 netduino2plus.build.mcu=cortex-m4
 netduino2plus.build.f_cpu=168000000L
 netduino2plus.build.core=maple
-netduino2plus.build.extra_flags=-DMCU_STM32F406VG -mthumb -DSTM32_HIGH_DENSITY -DSTM32F2 -DSTM32F4 -DBOARD_discovery_f4
+netduino2plus.build.extra_flags=-DMCU_STM32F406VG -mthumb -DSTM32_HIGH_DENSITY -DSTM32F2 -DSTM32F4 -DBOARD_discovery_f4 -DARDUINO_STM32F4_NETDUINO2PLUS
 netduino2plus.build.ldscript=ld/jtag.ld
 netduino2plus.build.variant=discovery_f407
 netduino2plus.build.variant_system_lib=lib_f407.a