CMake: Set TARGET_NAME macro

The C/C++ macro `TARGET_NAME` allows code to print the current
target's name, e.g. in error reporting when a fault happens. This
macro is exported by Mbed CLI 1 and used by mbed_error.c, so for
compatibility we let CMake set the same name of macro.

Fixes ARMmbed/mbed-tools#313

CMakeLists.txt

@@ -101,6 +101,7 @@ if(${CMAKE_CROSSCOMPILING})
 
     target_compile_definitions(mbed-core
         INTERFACE
+            TARGET_NAME=${MBED_TARGET}
             ${MBED_TARGET_DEFINITIONS}
             ${MBED_CONFIG_DEFINITIONS}
     )