CMSIS Pack: Use pdsc-relative path for resource files (#1461)

This patch will try to read resource files (e.g. FLMs) using
relative path to the pdsc file, which is used when some hardware vendors
places their pdscs under subdirectory of a pack archive, and somehow
valid for Keil MDK.

Fixes #1460

Signed-off-by: Yilin Sun <imi415@imi.moe>

pyocd/target/pack/cmsis_pack.py

@@ -129,6 +129,13 @@ def get_file(self, filename) -> IO[bytes]:
             opened (due to particularities of the ZipFile implementation).
         """
         filename = filename.replace('\\', '/')
+
+        # Some vendors place their pdsc in some subdirectories of the pack archive,
+        # use relative directory to the pdsc file while reading other files.
+        pdsc_base = self._pdscName.rsplit('/', 1)
+        if len(pdsc_base) == 2:
+            filename = f'{pdsc_base[0]}/{filename}'
+
         return io.BytesIO(self._pack_file.read(filename))
 
 class CmsisPackDescription: