Simplify linking/building objects in subdirectories

Simplify linking/building objects in subdirectories

New-style Makefiles have a nice way of declaring objects
which need to be built either built-in or as modules:

        obj-$(CONFIG_EEPRO100) += eepro100.o

However, handling objects in subdirectories, which need to be
built and linked is not as nice:

        subdir-$(CONFIG_E100) += e100

        ifeq ($(CONFIG_E100),y)
          obj-y += e100/built-in.o
        endif

This means we descend into the subdirectory when building
vmlinux / modules, depending on CONFIG_XXX. When we are building
vmlinux we also need to link whatever has been built in the
subdirectory, so we add it to $(obj-y) at the appropriate place
(link order is important).

Now, the extension below allows to rewrite the second case into

         obj-$(CONFIG_E100) += e100/

which looks much nicer ;-) Existing behavior is not changed, and the
only prerequisite to using the extension above is that the O_TARGET in
the subdir is named "built-in.o".