[DFS] Update DFS code.

1. Add extern "C" for cplusplus in DFS.
2. Export more symbol in DFS.

components/dfs/include/dfs_posix.h

@@ -30,6 +30,10 @@
 #include <dfs_file.h>
 #include <dfs_def.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #ifndef RT_USING_NEWLIB
 #define O_RDONLY    DFS_O_RDONLY
 #define O_WRONLY    DFS_O_WRONLY
@@ -79,6 +83,7 @@
 
 #if defined(__CC_ARM)
 #include <stdio.h>
+#include <stdlib.h>
 #elif defined(_MSC_VER)
 #include <stdio.h>
 #else
@@ -126,5 +131,8 @@ int rmdir(const char *path);
 int chdir(const char *path);
 char *getcwd(char *buf, size_t size);
 
+#ifdef __cplusplus
+}
 #endif
 
+#endif

components/dfs/src/dfs.c

@@ -284,6 +284,7 @@ const char *dfs_subdir(const char *directory, const char *filename)
 
     return dir;
 }
+RTM_EXPORT(dfs_subdir);
 
 /**
  * this function will normalize a path according to specified parent directory
@@ -410,5 +411,7 @@ up_one:
 
     return fullpath;
 }
+RTM_EXPORT(dfs_normalize_path);
+
 /*@}*/