FROMGIT: kbuild: make multi_depend work with targets in subdirectory

Precisely speaking, when you get the stem of the path, you should use
$(patsubst $(obj)/%,%,...) instead of $(notdir ...).

I do not see this usecase, but if you create a composite object in a
subdirectory, the Makefile should look like this:

   obj-$(CONFIG_FOO) += dir/foo.o
   dir/foo-objs      := dir/foo1.o dir/foo2.o

The member objects should be assigned to dir/foo-objs instead of
foo-objs.

This syntax is more consistent with commit 54b8ae66 ("kbuild:
change *FLAGS_<basetarget>.o to take the path relative to $(obj)").

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

Bug: 175420575
(cherry picked from commit 1fe9c5794b2b57b33c84ffaa0eb56254f310ed54
  https://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git

 kbuild)

Change-Id: I541fb600d05ae9be847718c869e0b95c0b540ff9
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>