cmake: Deprecate 'set_conf_file'

In-tree, all usage of 'set_conf_file' is implementing the same
pattern (with some inconsequential differences).

To reduce copy-paste and get closer to dropping support for
'set_conf_file', we include this pattern into the built-in set of
patterns.

'set_conf_file' is causing issues for future extensions,
e.g. multi-image support, and is also the source of a lot of
duplicated code.

Note support for 'set_conf_file' is not dropped, it is only marked as
deprecated.

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