[LWIP] Add lwip 2.0.2 TFTP server to SConscript.

components/net/lwip-2.0.2/SConscript

@@ -79,6 +79,7 @@ if GetDepend(['RT_LWIP_PPP']):
 # For testing apps
 if GetDepend(['RT_USING_NETUTILS']):
     src += Glob('./apps/*.c')
+    stc += Glob('./src/apps/tftp/*.c')
 
 group = DefineGroup('lwIP', src, depend = ['RT_USING_LWIP', 'RT_USING_LWIP202'], CPPPATH = path)