add gd32407v-start

bsp/gd32/gd32407v-start/board/SConscript

@@ -19,7 +19,9 @@ if rtconfig.CROSS_TOOL == 'gcc':
     src += [startup_path_prefix + '/GD32F4xx_HAL/CMSIS/GD/GD32F4xx/Source/GCC/startup_gd32f4xx.S']
 elif rtconfig.CROSS_TOOL == 'keil':
     src += [startup_path_prefix + '/GD32F4xx_HAL/CMSIS/GD/GD32F4xx/Source/ARM/startup_gd32f4xx.s']
-
+elif rtconfig.CROSS_TOOL == 'iar':
+    src += [startup_path_prefix + '/GD32F4xx_HAL/CMSIS/GD/GD32F4xx/Source/IAR/startup_gd32f4xx.s']
+    
 CPPDEFINES = ['GD32F407xx']
 group = DefineGroup('Drivers', src, depend = [''], CPPPATH = path, CPPDEFINES = CPPDEFINES)