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

[Raul Rangel (Code Review)]

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

Change subject: lib: Add a mutex
......................................................................

Patch Set 4:

(1 comment)

File src/include/mutex.h:

https://review.coreboot.org/c/coreboot/+/59320/comment/05e44a67_6db7c26f 
PS2, Line 11: void mutex_unlock(struct mutex *mutex);
> Done

Hrmm, so adding the guards is actually a little tricky. We would also need to duplicate the rules into the Makefile. Should we just add the `else if` to keep the rules simple in the Makefile? A no-op function will get garbage collected anyway.