add DFS_ROMFS_ROOT dependence on ROMFS source file.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1664 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/dfs/SConscript

@@ -19,8 +19,9 @@ filesystems/elmfat/ff.c
 # DFS-ROMFS options
 romfs = Split("""
 filesystems/romfs/dfs_romfs.c
-filesystems/romfs/romfs.c
 """)
+if not GetDepend('DFS_ROMFS_ROOT'):
+    romfs = romfs + 'filesystems/romfs/romfs.c'
 
 # DFS-DeviceFS options
 devfs = Split("""