Kbuild: Support nested composite objects

Kbuild composite object only supports one level of composite objects.
That is, a composite object may only be composed of real compilable
objects. A composite object may not itself be composed of other
composite objects.

As a simple example, the following Kbuild description is not supported:

bar-a-y := a/bar0.o a/bar1.o
bar-b-y := b/bar2.o b/bar3.o

foo-objs := bar-a.o bar-b.o

obj-m += foo.o

Add such support by recursively searching for composite objects and
listing them in $(multi-used-*) and $(real-obj-*).

Change-Id: I59df994237e27bf54d11034ff871442938050313
Signed-off-by: Elliot Berman <eberman@codeaurora.org>
Signed-off-by: Jeff Johnson <jjohnson@codeaurora.org>