Fixes ether initialization

We were unable to initialize more than one ethernet card.  Whichever was
linked in first would have its reset method run each time.

This ethernet init needs work anyways - seems like a lot of extra allocs
and resets.
Support for building directories outside the repo

If you have a directory, say a custom device, that you want compiled and
linked into the kernel, you can either set up an environment variable or
simply pass a variable to make:

$ export AKAROS_EXTERNAL_DIRS="../some-path/outside-repo/"
$ make

Run make from the usual place, in the Akaros repo.  Kbuild will build
that directory and link it into the final kernel binary.

You can have more than one directory.  They must be space separated.
You can do something like:

$ AKAROS_EXTERNAL_DIRS=$AKAROS_EXTERNAL_DIRS" ~/another-dir"

All directories must have a Kbuild file, and must produce at least one
.o file.  Whatever CONFIG variables are in the main kernel work here,
though I didn't build in Kconfig support.

Whatever functions you have in external directories won't get called
automatically.  Use linker function macros to get your initialization
routines run, and hopefully that's enough.  It should be for most plan9
style devices.