Instead of having to guess the PC where the SP was sampled, always take
both. This allows "seamless" stack decoding for both serial and xflash
dumps, since we don't have to guess which function generated the dump.
Make the core functions (doing the sampling) be ``noinline`` as well,
so that they always have valid frame.
Save SP which is closest to the crash location, which simplifies
debugging. For serial_dump, write SP just before the dump.
For xfdump, save SP in the dump header.
This makes xfdump_dump and xfdump_full_dump_and_reset() equivalent for
stack debugging.
As suggested by @3d-gussner, announce to the host that a dump is
available for retrieval using an action "dump_available".
Any kind of dump is announced (even if manually triggered).
To avoid reading from xflash twice, remove some duplication and return
the crash reason directly in xfdump_check_state().
