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

[Raul Rangel (Code Review)]

Attention is currently required from: Nico Huber, Tim Wawrzynczak, Angel Pons, Julius Werner, Kyösti Mälkki, Rob Barnes, 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 14:

(2 comments)

Patchset:

PS13:
> Let's not drag Google product schedules into this, that should not be relevant for an upstream desig […]

Ping Kyösti

File src/lib/mutex.c:

https://review.coreboot.org/c/coreboot/+/59320/comment/000003f9_2adb739d 
PS13, Line 43: 	assert(_mutex_is_locked_smp(mutex));
> In test environments, `_mutex_is_locked_smp(mutex)` could be evaluated twice, first in the if-block  […]

Are you just asking that I read the value into a variable? That's seems unnecessary here.