Minip work in progress

General usage:
    - include <minip.h>
    - Set up a callback for tx and rx with the IP stack. Zybo
      target does this already in target_init:

    minip_init(gem_send_raw_pkt, NULL, gem_rx_cb, NULL);
    gem_set_callback(minip_rx_driver_callback);

    This wires the gem driver to minip and sets up gem_rx_cb
    as the callback for udp packets, found in target.c.

    - To send packets first create an fd with:
        minip_fd_t *fd = minip_open(uint8_t addr[4], uint16_t port);

        then send with:
        send(fd, buf, len);

        Close / free the fd with minip_close(fd) when done with it.

    - ICMP ping /ARP are handled by the IP stack

Caveats:

    - Only seem to be getting 22-35 MB/s, so GIGE isn't properly working yet
    - When sending data full duplex there seems to be a way to jam tx.
        - Disabling ARP in minip.h can help this somewhat