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>

lib/Kconfig

@@ -44,6 +44,9 @@ config REGEX
 	  regex support to some commands, for example "env grep" and
 	  "setexpr".
 
+config LIB_RAND
+	bool
+
 source lib/rsa/Kconfig
 
 menu "Hashing Support"