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>

lib/hashtable.c

@@ -14,6 +14,7 @@
 
 #include <errno.h>
 #include <malloc.h>
+#include <sort.h>
 
 #ifdef USE_HOSTCC		/* HOST build */
 # include <string.h>