tfm-post-build: Don't capture subprocess stdout

subprocess.PIPE is used to enable the parent process to communicate with
the subprocess via pipes, which mean all stdout and stderr messages are
captured and returned as part of Popen.communicate's result tuple.

In our case, we want to display the error messages on the console, so we
don't need to capture the output from stdout.

Example of a typical error message before this change:
```
Traceback (most recent call last):
  File "platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/generate_mbed_image.py", line 197, in <module>
    sign_and_merge_tfm_bin(args.tfm_target, args.target_path, args.non_secure_bin, args.secure_bin)
  File "platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/generate_mbed_image.py", line 81, in sign_and_merge_tfm_bin
    " secure binary, Error code: " + str(retcode))
Exception: Unable to sign musca_b1 secure binary, Error code: 1
```

Example of the error message after this change:
```
Traceback (most recent call last):
  File "/mbed-os/tools/psa/tfm/bin_utils/wrapper.py", line 13, in <module>
    import click
ModuleNotFoundError: No module named 'click'
Traceback (most recent call last):
  File "platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/generate_mbed_image.py", line 194, in <module>
    sign_and_merge_tfm_bin(args.tfm_target, args.target_path, args.non_secure_bin, args.secure_bin)
  File "platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/generate_mbed_image.py", line 80, in sign_and_merge_tfm_bin
    raise Exception("Unable to sign " + target_name +
Exception: Unable to sign musca_b1 secure binary, Error code: 1
```

This is a significant improvement as now you can see what the reason for
the failure was.

platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/generate_mbed_image.py

@@ -152,14 +152,11 @@ def find_bl2_size(configFile):
     return bl2_size
 
 def run_cmd(cmd, directory):
-
     popen_instance = subprocess.Popen(
         cmd,
-        stdout=subprocess.PIPE,
         stderr=subprocess.STDOUT,
         cwd=directory,
     )
-
     popen_instance.communicate()
     return popen_instance.returncode