kbuild: deb-pkg: make debian source package working again

Since commit c5bf2efb ("kbuild: deb-pkg: fix binary-arch and clean
in debian/rules"), the source package generated by 'make deb-pkg' fails
to build.

I terribly missed the fact that the intdeb-pkg target may regenerate
include/config/kernel.release due to the following in the top Makefile:

  %pkg: include/config/kernel.release FORCE

Restore KERNELRELEASE= option to avoid the kernel.release disagreement
between build-arch and binary-arch.

Fixes: c5bf2efb ("kbuild: deb-pkg: fix binary-arch and clean in debian/rules")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>