bootutil_public: allow to confirm padded image without copy-done flag

This patch is for fixing:
https://github.com/zephyrproject-rtos/zephyr/issues/34683



copy-done flag is used to design completely copied image.
It is used to recognize whether need to complete image upgrade.
If image is running then:
1) it was already verified and copied by MCUboot
   copy-done flag is set for sure.
2) or it was programmed directly using programming interface

For case 2) when image contains magic but not copy-done flag it was
impossible to confirm it. This patch removes needs of having copy-done
flag set while confirming. This make API behavior similar to situation
when the not-padded image was programmed.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>