Add ability to run robot main loop in a separate thread (#1895)

Default behavior is still to run the robot main loop in the main thread.

The ability to run the robot main loop in a separate thread and add a hook
for running a different function in the main thread is needed for simulation
GUI support on some platforms.

hal/src/main/native/include/hal/HAL.h

@@ -27,6 +27,7 @@
 #include "hal/HALBase.h"
 #include "hal/I2C.h"
 #include "hal/Interrupts.h"
+#include "hal/Main.h"
 #include "hal/Notifier.h"
 #include "hal/PDP.h"
 #include "hal/PWM.h"

hal/src/main/native/include/hal/Main.h

@@ -0,0 +1,67 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <stdint.h>
+
+#include "hal/Types.h"
+
+/**
+ * @defgroup hal_relay Main loop functions
+ * @ingroup hal_capi
+ * @{
+ */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Sets up the system to run the provided main loop in the main thread (e.g.
+ * the thread in which main() starts execution) and run the robot code in a
+ * separate thread.
+ *
+ * Normally the robot code runs in the main thread, but some GUI systems
+ * require the GUI be run in the main thread.
+ *
+ * To be effective, this function must be called before the robot code starts
+ * the main loop (e.g. by frc::StartRobot()).
+ *
+ * @param param parameter data to pass to mainFunc and exitFunc
+ * @param mainFunc the function to be run when HAL_RunMain() is called.
+ * @param exitFunc the function to be run when HAL_ExitMain() is called.
+ */
+void HAL_SetMain(void* param, void (*mainFunc)(void*), void (*exitFunc)(void*));
+
+/**
+ * Returns true if HAL_SetMain() has been called.
+ *
+ * @return True if HAL_SetMain() has been called, false otherwise.
+ */
+HAL_Bool HAL_HasMain(void);
+
+/**
+ * Runs the main function provided to HAL_SetMain().
+ *
+ * If HAL_SetMain() has not been called, simply sleeps until HAL_ExitMain()
+ * is called.
+ */
+void HAL_RunMain(void);
+
+/**
+ * Causes HAL_RunMain() to exit.
+ *
+ * If HAL_SetMain() has been called, this calls the exit function provided
+ * to that function.
+ */
+void HAL_ExitMain(void);
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+/** @} */