kbuild: Simplify DEBUG_INFO Kconfig handling

While playing with [1] I saw that the handling
of CONFIG_DEBUG_INFO can be simplified.

[1] https://patchwork.kernel.org/patch/11716107/



Signed-off-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>