[S] Change in flashrom[master]: dummyflasher: use new API to register shutdown function

[Alexander Goncharov (Code Review)]

Attention is currently required from: Edward O'Callaghan, Anastasia Klimchuk.

Alexander Goncharov has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/72408 )

Change subject: dummyflasher: use new API to register shutdown function
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Patch Set 2:

(1 comment)

File dummyflasher.c:

https://review.coreboot.org/c/flashrom/+/72408/comment/f5a78171_450db6e8 
PS2, Line 910:
> What if you try: […]

1) dummy_shutdown() has as an argument `void *data`, not `struct flashctx *flash`. It's just a pointer to the data. Of course, we can free the data, but we cannot make `rmst.(par/spi/opaque).data` equals NULL. If we do `emu_data = NULL`, it makes the _local_ pointer equals NULL.

2) When we talk about data ownership, the last owner of the data must release this data. Not the first, because it can bring us more problems.

I'll try to figure out what I can add to the unit tests.