[M2351] Support secure/non-secure combined build for PSA

Support secure/non-secure combined build for PSA target:
1.  In secure post-build, deliver built secure image to TARGET_NU_PREBUILD_SECURE
    directory.
2.  In non-secure post-build, merge non-secure image with secure image saved in
    TARGET_NU_PREBUILD_SECURE directory.
3.  In non-secure post-build, user can also drop secure image saved in
    TARGET_NU_PREBUILD_SECURE directory and provide its own by adding the line below
    in mbed_app.json:
    "target.extra_labels_remove": ["NU_PREBUILD_SECURE"]

Author: ccli8

targets/targets.json

@@ -8842,13 +8842,20 @@
         "inherits": ["NSPE_Target", "NU_PFM_M2351"],
         "core": "Cortex-M23-NS",
         "tfm.level": 1,
-        "extra_labels_add": ["M23_NS", "PSA", "TFM"],
+        "extra_labels_add": [
+            "M23_NS",
+            "PSA",
+            "TFM",
+            "NU_PREBUILD_SECURE"
+        ],
         "macros_add": [
             "MBED_TZ_DEFAULT_ACCESS=1",
             "CMSIS_NVIC_VIRTUAL",
             "MBEDTLS_PSA_CRYPTO_C"
         ],
         "components_add": ["FLASHIAP"],
+        "post_binary_hook": {"function": "M2351Code.merge_secure"},
+        "secure_image_filename": "tfm.hex",
         "overrides": {
             "secure-rom-start"      : "0x0",
             "secure-rom-size"       : "0x40000",
@@ -8864,14 +8871,20 @@
         "inherits": ["SPE_Target", "NU_PFM_M2351"],
         "core": "Cortex-M23",
         "tfm.level": 1,
-        "extra_labels_add": ["M23_S", "PSA", "TFM"],
+        "extra_labels_add": [
+            "M23_S",
+            "PSA",
+            "TFM"
+        ],
         "device_has_remove": ["SERIAL", "SERIAL_ASYNCH", "SERIAL_FC", "STDIO_MESSAGES"],
         "macros_add": [
             "DAUTH_CHIP_DEFAULT",
             "MBEDTLS_PSA_CRYPTO_C",
             "MBEDTLS_PSA_CRYPTO_SPM"
         ],
         "components_add": ["FLASHIAP"],
+        "deliver_to_target": "NU_PFM_M2351_NS",
+        "delivery_dir": "TARGET_NUVOTON/TARGET_M2351/TARGET_M23_NS/TARGET_NU_PFM_M2351_NS/TARGET_NU_PREBUILD_SECURE",
         "overrides": {
             "secure-rom-start"      : "0x0",
             "secure-rom-size"       : "0x40000",