kbuild: buildtar: remove warning for the default case

kbuild: buildtar: remove warning for the default case

Given KBUILD_IMAGE properly set in arch/*/Makefile, the default case
should work in most scenarios. The only oddity is the naming of the
copy destination, vmlinux-kbuild-${KERNELRELEASE}. Let's rename it
to vmlinuz-${KERNELRELEASE} because the kernel is often compressed.
Remove the warning to avoid unnecessary patch submissions when the
default case suffices.

Remove the x86 case, which is now equivalent to the default.

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