Fix imgtool import for CYTFM_064B0S2_4343W signing

Fixes: #14153

The target CYTFM_064B0S2_4343W is signed using `cysecuretools` which
depends on `imgtool` managed by `pip`. However, Arm Musca targets
requires a modified/wrapped version of the image tool copied from
trusted-firmware-m + MCUboot.

To avoid conflicts in the version of `imgtool` used, we should only
add the copied version of `imgtool` to python's system path for
Musca targets instead of doing it globally for all targets.

tools/targets/ARM_MUSCA.py

@@ -20,6 +20,11 @@ from os.path import abspath, basename, dirname, splitext, isdir
 from os.path import join as path_join
 import re
 import subprocess
+import sys
+
+# Add PSA TF-M binary utility scripts in system path
+TFM_SCRIPTS = abspath(path_join(dirname(__file__), '..', 'psa', 'tfm', 'bin_utils'))
+sys.path.insert(0, TFM_SCRIPTS)
 
 SCRIPT_DIR = dirname(abspath(__file__))
 MBED_OS_ROOT = abspath(path_join(SCRIPT_DIR, os.pardir, os.pardir))

tools/targets/__init__.py

@@ -26,17 +26,13 @@ import sys
 from collections import namedtuple
 from copy import copy
 from future.utils import raise_from
+from os.path import dirname, abspath, join
 from tools.resources import FileType
 from tools.settings import ROOT
 from tools.targets.LPC import patch
 from tools.paths import TOOLS_BOOTLOADERS
 from tools.utils import json_file_to_dict, NotSupportedException
 
-# Add PSA TF-M binary utility scripts in system path
-from os.path import dirname, abspath, join
-TFM_SCRIPTS = abspath(join(dirname(__file__), '..', 'psa', 'tfm', 'bin_utils'))
-sys.path.insert(0, TFM_SCRIPTS)
-
 
 __all__ = ["target", "TARGETS", "TARGET_MAP", "TARGET_NAMES", "CORE_LABELS",
            "CORE_ARCH", "HookError", "generate_py_target", "Target",