cmake: Align the DT build logs with Kconfig

Align how DT and Kconfig print log output to be more consistent.

For instance, Kconfig does not state which header file it generates,
so we remove this from DT.

Also, Kconfig states which sources it uses, which is useful to know
when there is a build failure, so we align with Kconfig by printing
the DT source and overlay files.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>