[dfs] add poll/select

1. Add poll/select APIs;
2. Add wait queue implementation;
3. Remove DFS_STATUS_* code and use errno;
4. Add pipe API;
5. Use a standalone fops in DFS;

components/dfs/SConscript

@@ -1,16 +1,24 @@
 from building import *
 
 # The set of source files associated with this SConscript file.
-src = Glob('src/*.c')
+src = Split('''
+src/dfs.c
+src/dfs_file.c
+src/dfs_fs.c
+src/dfs_posix.c
+''')
 cwd = GetCurrentDir()
 CPPPATH = [cwd + "/include"]
 
+if GetDepend('RT_USING_DFS_NET'):
+    src += ['src/poll.c', 'src/select.c']
+
 group = DefineGroup('Filesystem', src, depend = ['RT_USING_DFS'], CPPPATH = CPPPATH)
 
 if GetDepend('RT_USING_DFS'):
     # search in the file system implementation
     list = os.listdir(cwd)
-    
+
     for item in list:
         if os.path.isfile(os.path.join(cwd, item, 'SConscript')):
             group = group + SConscript(os.path.join(item, 'SConscript'))