don't segfault on unknown ethernet boards

eth_scan_one_addr() now returns an int indicating pass (it found a
known board at the specified address) or fail (no known board found at
that address).

eth_boards_scan() propagates this pass/fail signal up to anyio_find_dev(),
so it can report eth-board failures just like it does for pci, epp, etc.

This commit also fixes a harmless(?) bug where unknown ethernet boards
would cause mesaflash to miscount the number of discovered boards.