x86: add thread support

Thread support is added for the x86 architecture. Both
the local apic and the tsc udelay() functions have a
call to thread_yield_microseconds() so as to provide an
opportunity to run pending threads.

Change-Id: Ie39b9eb565eb189676c06645bdf2a8720fe0636a
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/3207
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
diff --git a/src/Kconfig b/src/Kconfig
index 4366bc8..308deaa 100644
--- a/src/Kconfig
+++ b/src/Kconfig
@@ -326,7 +326,7 @@
 	def_bool n
-	depends on TIMER_QUEUE
+	depends on TIMER_QUEUE && ARCH_X86
 	  Cooperative multitasking allows callbacks to be multiplexed on the
 	  main thread of ramstage. With this enabled it allows for multiple