kbuild: set correct abs_srctree and abs_objtree for package builds

When you run 'make rpm-pkg', the rpmbuild tool builds the kernel in
rpmbuild/BUILD, but $(abs_srctree) and $(abs_objtree) point to the
directory path where make was started, not the kernel is actually
being built. The same applies to 'make snap-pkg'. Fix it.

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