Add USE_TINYCRYPT build option to CMake files

Adds the USE_TINYCRYPT build option to the CMake build files. Default is
enabled.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 746b38a..39808f9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -185,7 +185,10 @@
 
 add_subdirectory(library)
 add_subdirectory(include)
-add_subdirectory(tinycrypt)
+
+if(USE_TINYCRYPT)
+    add_subdirectory(tinycrypt)
+endif()
 
 if(ENABLE_PROGRAMS)
     add_subdirectory(programs)