Change in coreboot[master]: lib: Add a mutex

[Julius Werner (Code Review)]

Attention is currently required from: Raul Rangel, Rob Barnes, Kyösti Mälkki, Karthik Ramasubramanian.
Julius Werner has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/59320 )

Change subject: lib: Add a mutex
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Patch Set 2:

(1 comment)

File src/include/mutex.h:

https://review.coreboot.org/c/coreboot/+/59320/comment/1c8343a6_ffaee121 
PS2, Line 11: void mutex_unlock(struct mutex *mutex);
These should be static inline no-ops when neither SMP nor threading is enabled.