Update SConstruct

Update SConstruct fix compilation error

Author: liuduanfei

bsp/imx6ul/SConstruct

@@ -20,6 +20,7 @@ env = Environment(tools = ['mingw'],
 	AR = rtconfig.AR, ARFLAGS = '-rc',
 	LINK = rtconfig.LINK, LINKFLAGS = rtconfig.LFLAGS)
 env.PrependENVPath('PATH', rtconfig.EXEC_PATH)
+env['ASCOM'] = env['ASPPCOM']
 
 Export('RTT_ROOT')
 Export('rtconfig')