net: Fix NET_RANDOM_ETHADDR dependencies

NET_RANDOM_ETHADDR depends on lib/rand.c. This patch adds dependency to
Kconfig to ensure that library is also compiled.

Remove the definitions from Blackfin boards' include/configs.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/net/Kconfig b/net/Kconfig
index 524b7e4..9a9846e 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -10,6 +10,7 @@
 
 config NET_RANDOM_ETHADDR
 	bool "Random ethaddr if unset"
+	select LIB_RAND
 	help
 	  Selecting this will allow the Ethernet interface to function
 	  even when the ethaddr variable for that interface is unset.