Group knowledge about building objects and linking them together
instead of splitting it between the top-level Makefile and
the per-directory Makefiles.

This change tries to keep the link order the same as it was, at least
as far as possible. One critical point is that we now link
the pcmcia / wireless netdrivers before the pcmcia subsystem.
However, the pcmcia subsystem is initialized via a late_initcall(),
so this shouldn't make a difference.

drivers/net/Makefile looks a bit long now, but that can be improved
a lot, patch yet to come.