common: Move random-number functions into their own header

Create a new rand.h header file and move functions into it, to reduce
the size of common.h

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

net/link_local.c

@@ -14,6 +14,7 @@
 #include <common.h>
 #include <env.h>
 #include <net.h>
+#include <rand.h>
 #include "arp.h"
 #include "net_rand.h"