common: Move sorting functions to their own header file

These don't need to be in common.h so move them out into a new header.
Also add some missing comments.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/fs/yaffs2/yaffs_qsort.c b/fs/yaffs2/yaffs_qsort.c
index b463569..32c767f 100644
--- a/fs/yaffs2/yaffs_qsort.c
+++ b/fs/yaffs2/yaffs_qsort.c
@@ -5,6 +5,7 @@
  */
 
 #include "yportenv.h"
+#include <sort.h>
 /* #include <linux/string.h> */
 
 /*