Fix loading freeze when netconsole is active

Netconsole calls eth_halt() before giving control to another operating
system.
But the state machine of netconsole don't take it into account.
Thus, netconsole calls network functions of an halted network device,
making the whole system freeze.
Rather than modifying the state machine of netconsole, we just unregister
the current network device before booting. It does work because
nc_send_packet() verifies that the current network device is not null.

Signed-off-by: Frédéric Leroy <fredo@starox.org>