SWD flashing slot 0 partially results in unwanted behavior

[d3zd3z]

If, after performing upgrades through mcuboot (writing to slot 1, rebooting, etc), a hardware tool, such as SWD, or JTAG is used to write a new image to slot 0, and that tool does not erase the entire partition, it results in unwanted behavior.

This is most noticeable if the image has not been maked as "Image OK", a subsequent boot with a replaced image can result in even a crash depending on the size of the image. In any case, MCUboot will attempt to revert the swap.

This originated as #zephyrproject-rtos/zephyr#37495, although it appears to entirely be an MCUboot issue.

The easiest workaround is to add --pad to the imgtool.py invocation so that the entire partition is written. Another solution is to always erase the entire flash with the jtag tool before adding new images.

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