[BSP] update MDK project file for STM32F4

Author: BernardXiong

bsp/stm32f40x/SConstruct

@@ -8,7 +8,12 @@ else:
     RTT_ROOT = os.path.normpath(os.getcwd() + '/../..')
 
 sys.path = sys.path + [os.path.join(RTT_ROOT, 'tools')]
-from building import *
+try:
+    from building import *
+except:
+    print 'Cannot found RT-Thread root directory, please check RTT_ROOT'
+    print RTT_ROOT
+    exit(-1)
 
 TARGET = 'rtthread-stm32f4xx.' + rtconfig.TARGET_EXT