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/macb.c b/drivers/net/macb.c
index aa39284..08bebf7 100644
--- a/drivers/net/macb.c
+++ b/drivers/net/macb.c
@@ -40,6 +40,7 @@
  */
 
 #include <net.h>
+#include <netdev.h>
 #include <malloc.h>
 
 #include <linux/mii.h>