CMake: Use CMAKE_MODULE_PATH to locate nuvoton post build hook

We were previously relying on the global MBED_PATH variable to provide
the root for the include path to the post build hooks. We can't
guarantee that MBED_PATH will be set by any application building us, so
use CMAKE_MODULE_PATH (which we always set) like we do for the other
post-build hooks.

Author: rwalton-arm

CMakeLists.txt

@@ -25,7 +25,7 @@ project(mbed-os)
 
 # Add all paths to the list files within Mbed OS
 list(APPEND CMAKE_MODULE_PATH
-    "${mbed-os_SOURCE_DIR}/platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts;${mbed-os_SOURCE_DIR}/targets/TARGET_Cypress/scripts;${mbed-os_SOURCE_DIR}/targets/TARGET_NXP/scripts"
+    "${mbed-os_SOURCE_DIR}/platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts;${mbed-os_SOURCE_DIR}/targets/TARGET_Cypress/scripts;${mbed-os_SOURCE_DIR}/targets/TARGET_NXP/scripts;${mbed-os_SOURCE_DIR}/targets/TARGET_NUVOTON/scripts/"
 )
 
 add_subdirectory(extern)

targets/TARGET_NUVOTON/TARGET_M2354/TARGET_TFM/TARGET_NU_M2354/COMPONENT_TFM_S_FW/CMakeLists.txt

@@ -1,7 +1,7 @@
 # Copyright (c) 2020-2021 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-include(${MBED_PATH}/targets/TARGET_NUVOTON/scripts/mbed_set_post_build_nuvoton.cmake)
+include(mbed_set_post_build_nuvoton)
 
 target_link_libraries(mbed-m2354-tfm
     INTERFACE

targets/TARGET_NUVOTON/scripts/mbed_set_post_build_nuvoton.cmake

@@ -1,7 +1,7 @@
 # Copyright (c) 2021 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-include(${MBED_PATH}/tools/cmake/mbed_set_post_build.cmake)
+include(mbed_set_post_build)
 
 #
 # Sign TF-M secure and non-secure images and combine them with the bootloader