cmake: remove _if_kconfig() functions

This set of functions seem to be there just because of historical
reasons, stemming from Kbuild. They are non-obvious and prone to errors,
so remove them in favor of the `_ifdef()` ones with an explicit
`CONFIG_` condition.

Script used:

git grep -l _if_kconfig | xargs sed -E -i
"s/_if_kconfig\(\s*(\w*)/_ifdef(CONFIG_\U\1\E \1/g"

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>