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/cm-bf537u.h b/include/configs/cm-bf537u.h
index 34ce75b..88c9982 100644
--- a/include/configs/cm-bf537u.h
+++ b/include/configs/cm-bf537u.h
@@ -71,7 +71,7 @@
 #define CONFIG_HOSTNAME		cm-bf537u
 /* Uncomment next line to use fixed MAC address */
 /* #define CONFIG_ETHADDR	02:80:ad:20:31:e8 */
-
+#define CONFIG_LIB_RAND
 
 /*
  * Flash Settings