kbuild: deb-pkg: add CONFIG_ prefix to kernel config options

This might be a kind of bike-shed, but I personally prefer grep'able
code.

I often do 'git grep CONFIG_FOO' instead of 'git grep FOO' when I
want to know where that CONFIG option is used.

This makes code longer, but I hope this is acceptable level.

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