CMakelists: conditionally add subdir if ARDUINO en

* Conditionally include sub directories specific to the
ARDUINO_API module in CMAkelists
* Solve zephyrproject-rtos#46

Signed-off-by: Dhruva Gole <goledhruva@gmail.com>

Author: DhruvaG2000

CMakeLists.txt

@@ -1,5 +1,8 @@
 # SPDX-License-Identifier: Apache-2.0
 
+if (CONFIG_ARDUINO_API)
 add_subdirectory(cores)
 add_subdirectory(variants)
 add_subdirectory(libraries)
+endif()
+