fs: Rename fs.h to fs_legacy.h

A new filesystem interface is coming, so rename the current header to
indicate that it is the old one.

Signed-off-by: Simon Glass <sjg@chromium.org>

cmd/test.c

@@ -5,7 +5,7 @@
  */
 
 #include <command.h>
-#include <fs.h>
+#include <fs_legacy.h>
 #include <log.h>
 #include <vsprintf.h>