pid: zone split out builders

To enable unit-testing, split the builders into their own
object files.

Tested: Ran on quanta-q71l board and it behaved as expected.

Change-Id: I92168ec9ae4946d12328e9c0b94a36bb89d0f718
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/pid/pidthread.hpp b/pid/pidthread.hpp
index 670d558..37c416a 100644
--- a/pid/pidthread.hpp
+++ b/pid/pidthread.hpp
@@ -3,4 +3,4 @@
 #include "pid/zone.hpp"
 
 /* Given a zone, run through the loops. */
-void PIDControlThread(std::shared_ptr<PIDZone> zone);
+void PIDControlThread(PIDZone* zone);