Change in coreboot[master]: thread: Add mutex

[Raul Rangel (Code Review)]

Attention is currently required from: Julius Werner, Eric Peers, Felix Held.
Raul Rangel has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/56230 )

Change subject: thread: Add mutex
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Patch Set 3:

(1 comment)

File src/lib/thread.c:

https://review.coreboot.org/c/coreboot/+/56230/comment/ff0bd480_1c97b72f 
PS2, Line 406: 		thread_yield_microseconds(10);
> do we need an eventual timeout here to catch improper unlocks?

I had that thought, but wanted to avoid adding a return value to this method.