At this point we will start adding nesting behavior to other files
than Makefile.target.  Because Makefile.objs is included by
Makefile.target, it is simpler to move the processing of
subdirectories there.

To enable this, only add per-target files to obj-y.  Use a separate
variable for the linker dependencies, all-obj-y.  This variable includes
obj-y and also all objects that are taken from other directories.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>