imgtool: sign.py :--confirm parameter is not taken in account if --overwrite-only mode is set

[grevaillot]

Generating pre-confirmed binaries does not work if overwrite only mode is selected : option is silently ignored and files are generated.

cf : https://github.com/mcu-tools/mcuboot/blob/3a33d4faceb86abb2229d4dd09486514df4fac43/scripts/imgtool/image.py#L384 & below

Condition was introduced with the initial feature commit - https://github.com/mcu-tools/mcuboot/commit/0ce958e7095bb7d4a95efe5f65992b5804307a5f - but, although I understand the initial logic, we're using the confirmation mecanism in overwrite_only configuration to run some validations post upgrade, and need confirmed images for initial flashing.

Maybe some error could be thrown if i overlooked something and that test is mandatory - i scratched my head for a while to understand why freshly flashed images were not confirmed :)