massbuild: support for building kfreebsd kernel udebs added

[Frans Pop <elendil@planet.nl>]

It looks to work correctly (even on a linux host), but the resulting udebs 
have not been checked in detail.

Running depmod is simply skipped; AFAICT it's not relevant for kfreebsd 
kernels. If some other form of module dependency calculation is used, then 
it probably needs more work.

To build kfreebsd udebs for all (both) kfreebsd arches use:
$ ./massbuild kbuild --bsd

Or for a specific architecture (the kfreebsd- prefix is optional):
$ ./massbuild kbuild --bsd amd64
$ ./massbuild kbuild --bsd kfreebsd-i386

Other operations (kclean/krelease) and options should all work analogous to 
massbuilding for linux.

Cheers,
FJP