scripts: kconfig.py: Detect all assignments to promptless symbols

Assigning to promptless symbols has no effect.

Previously, the only check was for whether the value assigned to a
symbol matched its final value. This misses cases where a promptless
symbol is assigned to and just happens to get the assigned-to value as
its final value.

Instead, detect whether configuration files are being merged (by
checking if zephyr/.config already exists), and explicitly check for
assignments to promptless symbols in that case. We can't do it when
zephyr/.config already exists (and is being loaded), because it includes
values for promptless symbols as well.

With the no-prompt check moved out, also use a more specific message for
it, and remove stuff related to prompts elsewhere. Shorten messages a
bit at the same time, and add two warn() and err() helpers.

Fixes: #20697

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>