kbuild: skip sub-make for in-tree build with GNU Make 4.x

Commit 2b50f7ab ("kbuild: add workaround for Debian make-kpkg")
annoyed people who want to wrap the top Makefile with GNUmakefile
to customize it for their use.

On second thought, we do not need to run the sub-make for in-tree
build with Make 4.x because the 'MAKEFLAGS += -rR' issue only happens
on GNU Make 3.x.

With this commit, people will get back their workflow, and the Debian
make-kpkg will still work.

Fixes: 2b50f7ab

 ("kbuild: add workaround for Debian make-kpkg")
Reported-by: Andreas Schwab <schwab@suse.de>
Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Andreas Schwab <schwab@suse.de>
Tested-by: David Howells <dhowells@redhat.com>