CMake: Fix platform wait_ns greentea test Cmake

Mbed CLI 2 generates TARGET_ARM_FM macro as a string in mbed_config.cmake
under the MBED_TARGET_DEFINITIONS list. Fix the "if defined" check for
TARGET_ARM_FM by searching for the TARGET_ARM_FM string in the
MBED_TARGET_DEFINITIONS list. This allows us to skip or include the test from CMake.

platform/tests/TESTS/mbed_platform/wait_ns/CMakeLists.txt

@@ -3,7 +3,7 @@
 
 include(mbed_greentea)
 
-if(DEFINED TARGET_ARM_FM)
+if("TARGET_ARM_FM" IN_LIST MBED_TARGET_DEFINITIONS)
     set(TEST_SKIPPED "Wait ns test not supported.")
 endif()