core: prepare for generating multiple kconfig fragments

core: prepare for generating multiple kconfig fragments

We currently redirect the output of each helper function. This was nice
as long as we were generating single .mk and .in fragments.

But we are soon to need more .in fragments.

So, do the redirection inside the .in helpers.

We do not (currently) need to generate more than one .mk fragment, but
for consistency, do the redirection in the .mk helper too.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Vadim Kochan <vadim4j@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>