boot: bootutil: avoid full erase on BOOT_MAGIC_BAD in boot_set_next

[ofirshe]

Previously, boot_set_next() would perform a full slot erase when
BOOT_MAGIC_BAD was detected. This is no longer done, as the
decision on how to handle a corrupted image should be left to user code.

[ofirshe]

@de-nordic

[nordicjm]

this is from the user application side, the image would not be scrambled because this function does not run from MCUboot

[de-nordic]

this is from the user application side, the image would not be scrambled because this function does not run from MCUboot

Shouldn't we leave the decision what to do to a user code? User may want to just re-start DFU so will erase or attempt to erase the area anyway. We report here bad image, so user will know that there is a problem.

[ofirshe]

this is from the user application side, the image would not be scrambled because this function does not run from MCUboot

@nordicjm You're right - this function runs from the user application, so the image won't be scrambled at that point. What I meant is that the invalid image will be detected on the next reboot, and MCUboot will take the appropriate action.
I agree with @de-nordic that the decision on how to handle this should be left to the user, which is the motivation behind this PR. I've changed the commit message accordingly.