cmake: kconfig: use absolute files for merge_config_files

The `merge_config_files` list contains files with both relative and
absolute paths.
Each entry is checked and files with a relative path are converted to
absolute path. This creates a `merge_config_files_with_absolute_paths`
identical to the `merge_config_files` but with absolute path on each
entry.

Afterwards it is mixed whether `merge_config_files` or
`merge_config_files_with_absolute_paths` are used creating unnecessary
risk when the list with relative path files are used.

This commit sets the content of `merge_config_files` with updated
absolute paths files, and also updates all places to use this list.

This ensures that after path conversion then only the list with absolute
paths is used.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>