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.cpp b/pid/pidthread.cpp
index 2231390..d387ac9 100644
--- a/pid/pidthread.cpp
+++ b/pid/pidthread.cpp
@@ -26,7 +26,7 @@
 #include "sensors/sensor.hpp"
 
 
-static void ProcessThermals(std::shared_ptr<PIDZone> zone)
+static void ProcessThermals(PIDZone* zone)
 {
     // Get the latest margins.
     zone->updateSensors();
@@ -39,7 +39,7 @@
 }
 
 
-void PIDControlThread(std::shared_ptr<PIDZone> zone)
+void PIDControlThread(PIDZone* zone)
 {
     int ms100cnt = 0;
     /*