Pivos: Fork XBMC into a subshell

XBMC was running in the main init shell, which was also our serial
terminal.  Forking a subshell frees it up again.
Pivos: Wait for net devices to come up.

In switching our wifi drivers from built-in to kernel modules, the
device wouldn't be ready before init tried to start it.  Consequently,
we'd have no wifi unless S40network was rerun.  Block on "ifconfig
[interface] up" until it succeeds.
Pivos: Update our kernel for wifi modules