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 :

mcuboot/scripts/imgtool/image.py

Line 384 in 3a33d4f

if self.confirm and not self.overwrite_only:

& below

Condition was introduced with the initial feature commit - 0ce958e - 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 :)