We used to eth_open/eth_halt the network devices during NetLoopInit
which is called whenever the user enters a network command.
Change this behaviour so that the current network device gets opened
when making it the current one.
With this change it's always possible to send packages and we are able
to implement a new network stack in the next step.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>