kbuild: save overridden KERNELRELEASE in include/config/kernel.release

${KERNELRELEASE} is used as a part of the installation path.
(INSTALL_DTBS_PATH, MODLIB, etc.)

When KERNELRELEASE is overridden from the command line, it should be
saved in include/config/kernel.release, so that it will be consistently
used for the installation steps.

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