Pull request for the Extensions interface only (#655)

* Modify halsim to be able to load extension libraries if they are available.

It will read the list of libraries to try from the HALSIM_EXTENSIONS
environment variable.  Multiple libraries can be given if separated
by ';' (Windows) or ':' (Unix).

The library must have an 'HALSIM_InitExtension' method that returns >= 0 on success.

The library is expected to use the interface expressed by
  hal/src/src/main/native/include/MockData

* Add a simple halsim library that just prints robot values.

This makes a good test bed for cross platform purposes,
and provides the ultimate in light weight simulators.

This initial version only prints PWM values.

hal/src/main/native/sim/Extensions.cpp

@@ -0,0 +1,83 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2017 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "HAL/Extensions.h"
+
+#include <llvm/SmallString.h>
+#include <llvm/StringRef.h>
+
+#include "HAL/HAL.h"
+
+#if defined(WIN32) || defined(_WIN32)
+#include <windows.h>
+#else
+#include <dlfcn.h>
+#endif
+
+#if defined(WIN32) || defined(_WIN32)
+#define DELIM ';'
+#define HTYPE HMODULE
+#define DLOPEN(a) LoadLibrary(a)
+#define DLSYM GetProcAddress
+#define DLCLOSE FreeLibrary
+#else
+#define DELIM ':'
+#define HTYPE void*
+#define PREFIX "lib"
+#define DLOPEN(a) dlopen(a, RTLD_LAZY)
+#define DLSYM dlsym
+#define DLCLOSE dlclose
+#endif
+
+extern "C" {
+
+int HAL_LoadOneExtension(const char* library) {
+  int rc = 1;  // It is expected and reasonable not to find an extra simulation
+  HTYPE handle = DLOPEN(library);
+#if !defined(WIN32) && !defined(_WIN32)
+  if (!handle) {
+    llvm::SmallString<128> libraryName("lib");
+    libraryName += library;
+#if defined(__APPLE__)
+    libraryName += ".dylib";
+#else
+    libraryName += ".so";
+#endif
+    handle = DLOPEN(libraryName.c_str());
+  }
+#endif
+  if (!handle) return rc;
+
+  auto init = reinterpret_cast<halsim_extension_init_func_t*>(
+      DLSYM(handle, "HALSIM_InitExtension"));
+
+  if (init) rc = (*init)();
+
+  if (rc != 0) DLCLOSE(handle);
+  return rc;
+}
+
+/**
+ * Load any extra halsim libraries provided in the HALSIM_EXTENSIONS
+ * environment variable.
+ */
+int HAL_LoadExtensions(void) {
+  int rc = 1;
+  llvm::SmallVector<llvm::StringRef, 2> libraries;
+  const char* e = std::getenv("HALSIM_EXTENSIONS");
+  if (!e) return rc;
+  llvm::StringRef env{e};
+  env.split(libraries, DELIM, -1, false);
+  for (auto& libref : libraries) {
+    llvm::SmallString<128> library(libref);
+    rc = HAL_LoadOneExtension(library.c_str());
+    if (rc < 0) break;
+  }
+  return rc;
+}
+
+}  // extern "C"