overridden if MACHINE_LOADFILE_MACHDEP is defined.
This makes life much simpler in the face of the myriad of
different boot options for the evb* ports.
implement loadfile() in terms of it.
This allows clients to open a file once and "load" it multiple times (e.g.
first with COUNT_KERNEL, then with LOAD_KERNEL) without the side-effects
of multiple open calls.
