Introduce netdev.h header file and remove externs

This addresses all drivers whose initializers have already
been moved to board_eth_init()/cpu_eth_init().

Signed-off-by: Ben Warren <biggerbadderben@gmail.com>
diff --git a/drivers/net/bfin_mac.c b/drivers/net/bfin_mac.c
index 3ee5d96..c5e74b8 100644
--- a/drivers/net/bfin_mac.c
+++ b/drivers/net/bfin_mac.c
@@ -9,6 +9,7 @@
 #include <common.h>
 #include <config.h>
 #include <net.h>
+#include <netdev.h>
 #include <command.h>
 #include <malloc.h>