kbuild: compute the dtbs_install destination more simply

The 'dtbinst_root' is used to remember the root of the in-kernel dts
directory (i.e. arch/*/boot/dts), but it looks clumsy.

I prefer using two variables 'obj' and 'dst' to track the in-kernel
directory and the install destination, respectively.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>