Boot Loader: OFW network booting support

I'd like to drop support for OFW network booting.

These days, the only OpenFirmware machines are really old Mac G4 and G5
machines. Nothing else can use it (it used to be shared with sparc64
machines).

I'm in the process of reworking some of the disk code, which touches the
OFW disk code which would force me to write extra code for OFW since it
doesn't fit the pattern we have on other architectures very well.

So, rather than do a lot of work I can barely test (I might have an old G4
mac in my dad's old thing), I'm thinking about dropping the support,
especially since I don't think it's been used by anybody in a long time.

Before I do that, though, I'd see if anyone is actively using it...

Warner