ANDROID: kbuild: Search external devicetree path when running clean target

When running the clean target, kbuild is supposed to remove dtbs and
dtbos. However, kbuild only searches for those devicetree build
artifacts in the directory that the kernel binaries or kernel modules
are output to. This is not sufficient in cases where an external
devicetree is used, so include the external devicetree path when
searching for dtb and dtbo build artifacts.

Bug: 264602319
Fixes: 3d42cc9e ("ANDROID: kbuild: add support for compiling external device trees")
Change-Id: I45fdfdef09c3d57401d98e5db731273147d7d265
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>