[V3] Implementing the receive buffer for ofnet

[Stanislav Kholmanskikh]

Hi!

This is V3 of the series:

http://lists.gnu.org/archive/html/grub-devel/2016-04/msg00015.html

Changes since V2:

http://lists.gnu.org/archive/html/grub-devel/2016-12/msg00005.html

- Removed the patch which adds a check for error for grub_netbuff_reserve(nb, 2)
- Keep 'return 0' in search_net_devices() if we fail to allocate
  memory for txbuf or rcvbuf
- Removed the patch which frees memory on module unload.
- Put the ofnet_alloc_netbuf() wrapper into the patch enabling
  the receive buffer.

Thanks.