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>

drivers/bootcount/bootcount_fs.c

@@ -4,7 +4,7 @@
  */
 
 #include <bootcount.h>
-#include <fs.h>
+#include <fs_legacy.h>
 #include <mapmem.h>
 
 #define BC_MAGIC	0xbd