hm2_eth: note that only one board is supported
hm2_eth: Fix ARP pinning

 * Don't require specification of MAC address at all.  Instead,
   request the hardware address from the attached hostmot2 ethernet
   board.  (of all the ways to get the MAC address automatically,
   this seemed like the best one.  Alternate ways involved parsing
   /proc/net/arp; or using SIOCGARP which requires correctly filling
   in the interface name that the hostmot2 card is connected to, and
   also still requires sending a packet to the board)

 * When SIOCSARP (set arp entry) fails, report it; this will happen if
   you did not "sudo make setuid".

 * When an ARP entry was successfully pinned in init_net, delete it
   in close_net with SIOCDARP.