use environment variables for EFR SDK root (project-chip#25730)

* Use efr32 sdk root if available for building efr examples

* Add a comment

* Fix typo

* Add shell escape

* Use shell escape for the sdk root

* Restyle

---------

Co-authored-by: Andrei Litvin <andreilitvin@google.com>

scripts/build/builders/efr32.py

@@ -218,6 +218,12 @@ def __init__(self,
             self.extra_gn_options.append(
                 'sl_matter_version_str="v1.0-%s-%s"' % (branchName, shortCommitSha))
 
+        if "GSDK_ROOT" in os.environ:
+            # EFR32 SDK is very large. If the SDK path is already known (the
+            # case for pre-installed images), use it directly.
+            sdk_path = shlex.quote(os.environ['GSDK_ROOT'])
+            self.extra_gn_options.append(f"efr32_sdk_root=\"{sdk_path}\"")
+
     def GnBuildArgs(self):
         return self.extra_gn_options