CMake platform: remove MBED_PATH, use <PROJECT_NAME>_SOURCE_DIR

Author: 0xc0170

platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/mbed_set_post_build_tfm.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-os_SOURCE_DIR}/tools/cmake/mbed_set_post_build.cmake)
 
 #
 # Sign TF-M secure and non-secure images and combine them with the bootloader
@@ -17,7 +17,7 @@ function(mbed_post_build_tfm_sign_image
     set(mbed_target_name ${mbed_target})
     set(post_build_command
         COMMAND ${Python3_EXECUTABLE}
-            ${MBED_PATH}/platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/generate_mbed_image.py
+            ${mbed-os_SOURCE_DIR}/platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/generate_mbed_image.py
             --tfm-target ${tfm_target}
             --target-path ${target_path}
             --secure-bin ${secure_bin}