net: Make sure the ethaddr is updated in net_init()

NetConsole may call NetSendUDPPacket before NetLoop is called.  This
will cause the source MAC address (NetOurEther) to be wrong.  Instead
of only changing it in NetLoop, move it to NetLoopInit so that it is
also updated when net_init() is called (especially by nc_start()).

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reported-by: Michael Walle <michael@walle.cc>
Acked-by: Michael Walle <michael@walle.cc>