sensors: split building from manager object

To increase testability, split out the building of sensors
from the sensor manager.  And this further splits out building
from a configuration file.

Tested: Verified code continued to build and link.
Tested: Ran on quanta-q71l board and it behaved as expected.

Change-Id: Ib63a11e1107b1864c3c370bba2bd9ef2effa42f3
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/main.cpp b/main.cpp
index ed1b8c7..6dbd7d6 100644
--- a/main.cpp
+++ b/main.cpp
@@ -35,6 +35,8 @@
 /* Controllers & Sensors. */
 #include "interfaces.hpp"
 #include "pid/zone.hpp"
+#include "sensors/builder.hpp"
+#include "sensors/builderconfig.hpp"
 #include "sensors/manager.hpp"
 
 /* Threads. */