cmake: mcuboot: Flash confirmed file if available

In case a confirmed image gets generated, it should be used during
flash process.

If there is a valid application in second image slot, flashing
*.signed.hex will result in a unconfirmed image in first slot.
After reboot MCUboot will replace the unconfirmed image by swaping
the confirmed application from the second into the first slot.

Signed-off-by: David Schneider <schneidav81@gmail.com>