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/cpu/leon2/cpu.c b/cpu/leon2/cpu.c
index 5de1c52..d6e23b1 100644
--- a/cpu/leon2/cpu.c
+++ b/cpu/leon2/cpu.c
@@ -25,6 +25,7 @@
 #include <common.h>
 #include <watchdog.h>
 #include <command.h>
+#include <netdev.h>
 
 DECLARE_GLOBAL_DATA_PTR;
 
@@ -57,8 +58,6 @@
 
 /* ------------------------------------------------------------------------- */
 
-extern int greth_initialize(bd_t *bis);
-
 #ifdef CONFIG_GRETH
 int cpu_eth_init(bd_t *bis)
 {