Change in coreboot[master]: lib/thread: Add mutex

[Raul Rangel (Code Review)]

Attention is currently required from: Julius Werner, Eric Peers, Felix Held.
Hello build bot (Jenkins), Julius Werner, Felix Held,

I'd like you to reexamine a change. Please visit

https://review.coreboot.org/c/coreboot/+/56230

to look at the new patch set (#5).

Change subject: lib/thread: Add mutex
......................................................................

lib/thread: Add mutex

We need a way to protect shared resources. Since we are using cooperative
multitasking the mutex implementation is pretty trivial.

The delays are implemented using a timer queue. So the task with the
next expiration date will always be at the beginning of the queue.

If a task (A) wishes to yield control to the next task (B) that is
holding onto the mutex, without encountering a delay of its own
(i.e., udelay(0)), then we need to ensure the next task (B) is at the
head of the queue. We do this by using a `0 us` delay in mutex_lock. If
we were to use anything larger than a `0 us` delay, a `udelay(0)` by the
task (A) that just unlocked the mutex will most likely not schedule the
next task (B) that was holding onto the mutex.

BUG=b:179699789
TEST=Verify thread lock and unlock.

Signed-off-by: Raul E Rangel 
Change-Id: Ife1ac95ec064ebcdd00fcaacec37a06ac52885ff
---
M src/include/thread.h
M src/lib/thread.c
2 files changed, 31 insertions(+), 0 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/30/56230/5