[examples] move platform.h into examples (#2541)

The existing platform.h is really a part of the examples and not a part of
OpenThread core.  This commit moves the platform.h out of the public API
and into the examples directory.

tests/unit/Makefile.am

@@ -51,6 +51,7 @@ if OPENTHREAD_BUILD_TESTS
 
 AM_CPPFLAGS                                                         = \
     -DOPENTHREAD_FTD=1                                                \
+    -I$(top_srcdir)/examples/platforms                                \
     -I$(top_srcdir)/include                                           \
     -I$(top_srcdir)/src                                               \
     -I$(top_srcdir)/src/core                                          \

tests/unit/test_diag.cpp

@@ -27,11 +27,11 @@
  */
 
 #include <openthread/diag.h>
-#include <openthread/platform/platform.h>
 #include <openthread/platform/radio.h>
 
 #include "utils/wrap_string.h"
 
+#include "platform.h"
 #include "test_util.h"
 
 extern "C" void otTaskletsSignalPending(otInstance *)