kbuild: move no_new_adhoc_configs_check to "all" target command

I am going to move the build rule of u-boot.cfg.  Before that,
no_new_adhoc_configs_check must be tweaked to not depend on it.

The ad-hoc option check can be done at the end of build, along
with other checks.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>