lib/thread: Verify threads are initialized before yielding

In hardwaremain.c we call console_init before threads_initialize. Part
of setting up the uart requires calling udelay which then calls
thread_yield_microseconds. Since threads have not been set up, trying to
yield will result in bad things happening. This change guards the thread
methods by making current_thread return NULL if the structures have not
been initialized.

BUG=b:179699789
TEST=Ramstage no longer hangs with serial enabled

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: If9e1eedfaebe584901d2937c8aa24e158706fa43
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56318
Reviewed-by: Krystian Hebel <krystian.hebel@3mdeb.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/lib/thread.c

@@ -9,6 +9,8 @@
 #include <thread.h>
 #include <timer.h>
 
+static bool initialized;
+
 static void idle_thread_init(void);
 
 /* There needs to be at least one thread to run the ramstate state machine. */
@@ -40,6 +42,9 @@ static inline struct thread *cpu_info_to_thread(const struct cpu_info *ci)
 
 static inline struct thread *current_thread(void)
 {
+	if (!initialized)
+		return NULL;
+
 	return cpu_info_to_thread(cpu_info());
 }
 
@@ -265,6 +270,8 @@ void threads_initialize(void)
 		free_thread(t);
 	}
 
+	initialized = 1;
+
 	idle_thread_init();
 }