kbuild: deb-pkg: introduce is_enabled and if_enabled_echo to builddeb

I think is_enabled() and if_enable_echo() in scripts/package/mkdebian
are useful.

builddeb also has many repetitive greps over the kernel config, so I
borrowed the idea to clean it up.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>