net: use common rand()/srand() functions

Replace rand() with the functions from lib/. The link-local network code
stores its own seed, derived from the MAC address. Thus making it
independent from calls to srand() in other modules.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/include/common.h b/include/common.h
index e16c53e..d1dd65a 100644
--- a/include/common.h
+++ b/include/common.h
@@ -761,7 +761,9 @@
 #include <u-boot/crc.h>
 
 /* lib/rand.c */
-#ifdef CONFIG_RANDOM_MACADDR
+#if defined(CONFIG_RANDOM_MACADDR) || \
+	defined(CONFIG_BOOTP_RANDOM_DELAY) || \
+	defined(CONFIG_CMD_LINK_LOCAL)
 #define RAND_MAX -1U
 void srand(unsigned int seed);
 unsigned int rand(void);