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.