kbuild: move init/build-version to scripts/

At first, I thought this script would be needed only in init/Makefile.

However, commit 5db8face ("kbuild: Restore .version auto-increment
behaviour for Debian packages") and commit 1789fc91 ("kbuild:
rpm-pkg: invoke the kernel build from rpmbuild for binrpm-pkg")
revealed that it was actually needed for scripts/package/mk* as well.

After all, scripts/ is a better place for it.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>