netboot: Clean up the netboot.h header file.

There were a number of problems in this file. The compiler guard was wrong,
the netboot_entry symbol was undefined and never called, and the comment was
stale.

Change-Id: I2174db6c9380029b66f3cadeaf033c07a7050f99
diff --git a/src/net/netboot/netboot.h b/src/net/netboot/netboot.h
index b97912b..af9da23 100644
--- a/src/net/netboot/netboot.h
+++ b/src/net/netboot/netboot.h
@@ -17,15 +17,14 @@
  * MA 02111-1307 USA
  */
 
-#ifndef __NETBOOT_NETBOOT_H__
-#define __NETBOOT_NETBOOT_H__
+#ifndef __NET_NETBOOT_NETBOOT_H__
+#define __NET_NETBOOT_NETBOOT_H__
 
 #include <stddef.h>
 
 #include "net/ipv4/uip/uip.h"
 
-// argsfile takes precedence before args. All parameters can be NULL.
+// "args" can be NULL.
 void netboot(char *args, size_t args_size);
-int netboot_entry(void);
 
-#endif /* __NETBOOT_NETBOOT_H__ */
+#endif /* __NET_NETBOOT_NETBOOT_H__ */