修复当没有设置环境变量时scons -c出现的问题

Author: ScuDays

components/lwp/vdso/SConscript

@@ -24,7 +24,10 @@ src    +=Glob('kernel/*.S')
 
 
 PREFIX  = os.getenv('RTT_CC_PREFIX')
-CPP     = PREFIX + 'cpp'
+if PREFIX is None:
+  CPP = 'cpp'
+else:
+  CPP = PREFIX + 'cpp'
 CPPFLAGS= ' -E -P -x assembler-with-cpp'
 rtconfig.CPP = CPP
 rtconfig.CPPFLAGS = CPPFLAGS