add bootpath parser for open firmware

It enables net boot even when there is no bootp/dhcp server.

* grub-core/net/drivers/ieee1275/ofnet.c: Add grub_ieee1275_parse_bootpath and
call it at grub_ieee1275_net_config_real.
* grub-core/kern/ieee1275/init.c: Add bootpath to grub_ieee1275_net_config.
* include/grub/ieee1275/ieee1275.h: Likewise.

include/grub/ieee1275/ieee1275.h

@@ -70,8 +70,9 @@ struct grub_ieee1275_devalias
 };
 
 extern void (*EXPORT_VAR(grub_ieee1275_net_config)) (const char *dev,
-						     char **device,
-						     char **path);
+                                                     char **device,
+                                                     char **path,
+                                                     char *bootargs);
 
 /* Maps a device alias to a pathname.  */
 extern grub_ieee1275_phandle_t EXPORT_VAR(grub_ieee1275_chosen);