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/board/sl8245/sl8245.c b/board/sl8245/sl8245.c
index c853755..e66272e 100644
--- a/board/sl8245/sl8245.c
+++ b/board/sl8245/sl8245.c
@@ -24,6 +24,7 @@
 #include <common.h>
 #include <mpc824x.h>
 #include <pci.h>
+#include <netdev.h>
 
 int checkboard (void)
 {
@@ -67,8 +68,6 @@
 	pci_mpc824x_init(&hose);
 }
 
-extern int skge_initialize(bd_t *bis);
-
 int board_eth_init(bd_t *bis)
 {
 	int rc = 0;