Use sync packets to escape SWO pin transients?

[tmplt]

As documented, configuring a GPIO pin for SWO induces transient noise. Noise is not deterministic, and will be different across platforms. Recording the SWO pin from target start will then corrupt the bit buffer. Manual intervention is required to ensure that this transient is not recorded. Ideally, we should be able to filter this transient out.

The ITM standard allow the periodic generation of synchronization packets. Can we use these to escape the transient? Ideally, we should only require a single sync packet. With sync packet(s) expected, we can restore the bit buffer to a proper state when a sync packet is received and thus filter the transient out. This should be a configuration flag.