Change in flashrom[master]: buspirate_spi.c: Refactor singleton states into reentrant pattern

[Nico Huber (Code Review)]

Attention is currently required from: Edward O'Callaghan, Angel Pons, Anastasia Klimchuk.
Nico Huber has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/52958 )

Change subject: buspirate_spi.c: Refactor singleton states into reentrant pattern
......................................................................

Patch Set 2:

(2 comments)

File buspirate_spi.c:

https://review.coreboot.org/c/flashrom/+/52958/comment/cca037ab_c3e0c369 
PS2, Line 167: 	struct buspirate_spi_data *buspirate_data = data;
> I can probably create bp_commbuf as a local variable, alloc at the beginning and free at the end.

Absolutely, yes. As DEFAULT_BUFSIZE is small, it could also live on the
stack in most cases.
> bp_commbufsize is different, I can see why it needs to be in a spi data in flashctx.

AFAICT, it's only there to track the size of `bp_commbuf`. So they should
share a common fate.

I guess Angel is right, somebody tried to optimize things. Not in any way
that would gain us something, though. malloc() and free() are not expensive,
realloc() may be for bigger sizes. Compared to a single byte transferred
over a serial line, it's still nothing.

There's a third alternative. The size is eventually limited by what the
v2 command supports. Somewhere below max_data_read/write is given. We
could just allocate the maximum during init (256 + 5 + 2048 + 1). Define
this as BUFFER_SIZE and let the command functions check that their
parameters fit. Or a combination: use the stack during init, shutdown,
command_v1 and only allocate the buffer if we use command_v2.

https://review.coreboot.org/c/flashrom/+/52958/comment/da0c6345_aa3e5ecb 
PS2, Line 325: 	int bp_commbufsize = 0;
We could declare it as follows

/* 16 bytes data, 3 bytes control. */
    unsigned char commbuf[16 + 3];

Then use `sizeof(commbuf)` instead of DEFAULT_BUFSIZE below.

Something similar could be done to buspirate_shutdown(). All other
users of the `bp_commbuf` use buspirate_commbuf_grow() and can use
malloc()/free() instead. Hmm, maybe the v1 function should use the
stack as well?