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/bf537-stamp/bf537-stamp.c b/board/bf537-stamp/bf537-stamp.c
index a9b7a68..4567213 100644
--- a/board/bf537-stamp/bf537-stamp.c
+++ b/board/bf537-stamp/bf537-stamp.c
@@ -32,6 +32,7 @@
 #include <asm/io.h>
 #include <net.h>
 #include <asm/mach-common/bits/bootrom.h>
+#include <netdev.h>
 
 /**
  * is_valid_ether_addr - Determine if the given Ethernet address is valid
@@ -156,8 +157,6 @@
 
 #if defined(CONFIG_BFIN_MAC)
 
-extern int bfin_EMAC_initialize(bd_t *bis);
-
 int board_eth_init(bd_t *bis)
 {
 	return bfin_EMAC_initialize(bis);