nios2: enable CONFIG_NET_RANDOM_ETHADDR

Enable CONFIG_NET_RANDOM_ETHADDR to generate a random MAC address
when ETHADDR is not set.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Cc: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/configs/nios2-generic_defconfig b/configs/nios2-generic_defconfig
index 5f93394..f8d2821 100644
--- a/configs/nios2-generic_defconfig
+++ b/configs/nios2-generic_defconfig
@@ -10,3 +10,4 @@
 CONFIG_CMD_DHCP=y
 # CONFIG_CMD_NFS is not set
 CONFIG_CMD_PING=y
+CONFIG_NET_RANDOM_ETHADDR=y