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/mcffec.c b/drivers/net/mcffec.c
index 50d6508..f93cf59 100644
--- a/drivers/net/mcffec.c
+++ b/drivers/net/mcffec.c
@@ -32,6 +32,7 @@
 
 #include <command.h>
 #include <net.h>
+#include <netdev.h>
 #include <miiphy.h>
 
 #undef	ET_DEBUG