ANDROID: Kbuild: Support KBUILD_MIXED_TREE in Makefile.modfinal

When KBUILD_MIXED_TREE points to the output folder of another kernel's
build output, Kbuild can compile a complete kernel tree's modules
against that other kernel tree's vmlinux. This is useful when two
kernel trees exist: a "Generic Kernel Image" tree and a "device kernel"
tree.  Both trees are complete kernel source trees, and the "Generic
Kernel Image" should provide the kernel Image and device kernel tree
provides device driver modules.

To accomplish this, references to vmlinux in the device kernel
should point to the generic kernel's vmlinux and the device
kernel should skip compilation of built-in files.

Change ee1b16e8
adds support for KBUILD_MIXED_TREE
in Makefile and Makefile.modpost. This change adds support for
KBUILD_MIXED_TREE in Makefile.modfinal, where vmlinux is referenced.

Bug: 202075496

Change-Id: I9bb3ff34fd6b5f0510b87648b9942af25c903de0
Signed-off-by: Yifan Hong <elsk@google.com>