Allow settings to be expanded in arbitrary commands, such as

  kernel http://10.0.0.1/boot.php?uuid=${uuid}

Also add the "echo" command, as being the easiest way to test this
features.

Avoid returning errors for comments and empty lines.

names.

Add "dev" pointer in struct net_device to tie network interfaces back to a
hardware device.

Force natural alignment of data types in __table() macros.  This seems to
prevent gcc from taking the unilateral decision to occasionally increase
their alignment (which screws up the table packing).

command is linked in.