boot: Support bootstrapping for multiple images

[KarlKTorp]

Bootstrapping is currently not allowed for more than one image. But that seems to be unintentional according to https://github.com/mcu-tools/mcuboot/pull/826#issuecomment-707939624

I ran into the same issue trying to boot two images (for M7 and M4) on a i.MX RT1170 using zephyr. We solved this using the suggested change and hope it could be added to main.

With this it's possible to enable MCUmgr SMP within MCUBoot and update a device that only has the bootloader in flash.