Platform_extender: FT4222 library refactor

Updates to make the FT4222 platform extender more usable.

Previous implementation opened libft4222.so and loaded symbols as
needed.  This implementation removes dynamic loading of libft4222 in
favor of creating a new shared libary which links against libft4222.so.
The dynamic loading is now done in mraa.c.  One C method is exposed in
libmraa-platform-ft4222.so for finding/initializing an FT4222:

mraa_platform_t mraa_usb_platform_extender(mraa_board_t* board);

Mraa.c attempts to open this platform library and calls the
mraa_usb_platform_extender method.  If an ftdi4222 is connected, the
user gets added IO from the extender.  If no FT4222 device is connected,
continue as normal.

    * Create a new platform library for the FT4222
    * Expose only 1 C method from the library -
      mraa_usb_platform_extender
    * libmraa-platform-ft4222.so contains CXX code (as well as previous
      C code).  All *allocs have been removed in favor of global
      C++ stl containers.
    * Previously, the FT4222 would only initialize correctly if 2 ftdi
      devices existed.  Now, initialize FT4222 devices based on the
      device id.
    * Many fixes for various problems with the FT4222
    * Added unit test for platform extender (minimal functionality w/o hw)
    * Updated to FindFtd4222.cmake module to handle standard arguments
    * Removed CMAKE_C_FLAGS addition of -DFTDID2XX and -DFTDI4222 since
      these are NOT used anywhere in source.
    * Building the FTDI4222 shim requires libft4222.h which requires
      ftd2xx.h.  Updated CMakeLists.txt to require both when building
      the shim.

Signed-off-by: Noel Eck <noel.eck@intel.com>

tests/unit/CMakeLists.txt

@@ -20,9 +20,20 @@ target_link_libraries(api_common_hpp_unit GTest::GTest GTest::Main mraa)
 target_include_directories(api_common_hpp_unit PRIVATE "${CMAKE_SOURCE_DIR}/api")
 gtest_add_tests(api_common_hpp_unit "" AUTO)
 
+if (FTDI4222)
+    # Unit tests - Test platform extenders (as much as possible)
+    add_executable(platform_extender platform_extender/platform_extender.cxx)
+    target_link_libraries(platform_extender GTest::GTest GTest::Main mraa-platform-ft4222 dl)
+    target_include_directories(platform_extender PRIVATE "${PROJECT_SOURCE_DIR}/api"
+        "${PROJECT_SOURCE_DIR}/api/mraa"
+        "${PROJECT_SOURCE_DIR}/include")
+    gtest_add_tests(platform_extender "" AUTO)
+endif ()
+
 # Add a custom target for unit tests
 add_custom_target(tests-unit ALL
     DEPENDS
         api_common_h_unit
         api_common_hpp_unit
-    COMMENT "mraa unit test collection")
+        platform_extender
+        COMMENT "mraa unit test collection")

tests/unit/platform_extender/platform_extender.cxx

@@ -0,0 +1,31 @@
+#include <dlfcn.h>
+
+#include "gtest/gtest.h"
+#include "mraa/common.h"
+#include "mraa_internal_types.h"
+
+/* MRAA test fixture */
+class plaform_extender : public ::testing::Test {};
+
+/* Not much to test w/o hw - test a no-init case.
+ * Note, w/o a link to libmraa, this binary does NOT initialize mraa
+ * with the loader constructor attribute.  This is intended. */
+TEST_F(plaform_extender, test_no_init)
+{
+    mraa_board_t plat = {};
+
+    void* usblib = dlopen("libmraa-platform-ft4222.so", RTLD_LAZY);
+    ASSERT_NE((void*)NULL, usblib)
+        << "Failed to load libmraa-platform-ft4222.so, reason (" << dlerror()
+        << ")";
+
+    fptr_add_platform_extender add_ft4222_platform =
+        (fptr_add_platform_extender)dlsym(usblib, "mraa_usb_platform_extender");
+
+    ASSERT_NE((void*)NULL, add_ft4222_platform) << "Symbol 'add_ft4222_platform' "
+        << "does not exist in libmraa-platform-ft4222.so";
+
+    ASSERT_EQ(MRAA_ERROR_PLATFORM_NOT_INITIALISED, add_ft4222_platform(&plat))
+        << "Initialization returned a valid platform.  Make sure no Ft4222 "
+        << "device is connected";
+}