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>

env/common.c

@@ -11,6 +11,7 @@
 #include <command.h>
 #include <env.h>
 #include <env_internal.h>
+#include <sort.h>
 #include <linux/stddef.h>
 #include <search.h>
 #include <errno.h>