net: Move MAC-seeded rand out of bootp.c

Make the MAC-seeded random number generator available to /net in
general.  MAC-seeded rand will be needed by link-local as well, so
give it an interface.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/net/Makefile b/net/Makefile
index 0544f6b..5901046 100644
--- a/net/Makefile
+++ b/net/Makefile
@@ -31,6 +31,7 @@
 COBJS-$(CONFIG_CMD_DNS)  += dns.o
 COBJS-$(CONFIG_CMD_NET)  += eth.o
 COBJS-$(CONFIG_CMD_NET)  += net.o
+COBJS-$(CONFIG_BOOTP_RANDOM_DELAY) += net_rand.o
 COBJS-$(CONFIG_CMD_NFS)  += nfs.o
 COBJS-$(CONFIG_CMD_RARP) += rarp.o
 COBJS-$(CONFIG_CMD_SNTP) += sntp.o