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: NTzafrir Cohen <nvidia@cohens.org.il>
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1886,6 +1886,8 @@ endif
 
 else # KBUILD_EXTMOD
 
+filechk_kernel.release = echo $(KERNELRELEASE)
+
 ###
 # External module support.
 # When building external modules the kernel used as basis is considered