blackfin: replace bfin_gen_rand_mac() with eth_random_addr()

bfin_gen_rand_mac() uses __DATE__ as the seed for random ethernet
address. This makes the build non-deterministic.

In the first place, it should not be implemented as a Bfin-specific
function. Use eth_random_addr() instead.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Sonic Zhang <sonic.zhang@analog.com>
diff --git a/include/configs/bf537-minotaur.h b/include/configs/bf537-minotaur.h
index 156eeab..3bc364c 100644
--- a/include/configs/bf537-minotaur.h
+++ b/include/configs/bf537-minotaur.h
@@ -89,6 +89,7 @@
 /* Uncomment next line to use fixed MAC address */
 /* #define CONFIG_ETHADDR	02:80:ad:20:31:42 */
 
+#define CONFIG_LIB_RAND
 
 /*
  * Flash Settings