M2354: Pre-create cmake target mbed-m2354-tfm for custom_targets

For M2354 custom target, there is dilemma on cmake target mbed-m2354-tfm:
1. mbed-m2354-tfm is created in mbed-os M2354 cmake listfile, but may be used
   in custom_targets M2354 cmake listfile in advance.
2. Build system requires add_subdirectory(custom_targets) be placed in front of
   add_subdirectory(mbed-os).

To overcome above, mbed-m2354-tfm is pre-created on request and as singleton,
E.g.:
if(NOT TARGET mbed-m2354-tfm)
    add_library(mbed-m2354-tfm INTERFACE)
endif()

targets/TARGET_NUVOTON/TARGET_M2354/TARGET_TFM/CMakeLists.txt

@@ -1,7 +1,9 @@
 # Copyright (c) 2020-2021 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-add_library(mbed-m2354-tfm INTERFACE)
+if(NOT TARGET mbed-m2354-tfm)
+    add_library(mbed-m2354-tfm INTERFACE)
+endif()
 
 if("NU_M2354" IN_LIST MBED_TARGET_LABELS)
     add_subdirectory(TARGET_NU_M2354)