Makefile: Make kernelrelease target work with M=

That commit required the use of filechk_kernel.release for the
kernelrelease Makefile target. It is currently only being set when
KBUILD_EXTMOD is not set. Make sure it is set in that case as well.

Fixes: 1cb86b6c ("kbuild: save overridden KERNELRELEASE in include/config/kernel.release")
Signed-off-by: Tzafrir Cohen <nvidia@cohens.org.il>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>