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/lib/Kconfig b/lib/Kconfig
index 5b6cf3d..3c8de86 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -44,6 +44,9 @@
 	  regex support to some commands, for example "env grep" and
 	  "setexpr".
 
+config LIB_RAND
+	bool
+
 source lib/rsa/Kconfig
 
 menu "Hashing Support"