Bridged network boot

Bridged network boot

* SYNTAX
bridge=<bridgename>:<ethname>
If bridge without parameters, assume bridge=br0:eth0
* When <ethname> would be configured by network scripts, instead create a bridge named <bridgename> then add <ethname> to that bridge.
* Then $netif becomes <bridgename> instead of <ethname> and all existing scripts process netroot mount via this new $netif instead of <ethname>.
* Include a few test cases in NFS and NBD