Fix backwards-compat branch ref in legacy update method (#21)

# Description
Fix branch ref used in legacy update method

# Issues
Addresses test failure
#20

# Other Notes
<!-- Note any breaking changes, WIP changes, requests for input, etc.
here -->

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Co-authored-by: Daniel McKnight <[email protected]>

neon_phal_plugin_device_updater/__init__.py

@@ -125,6 +125,7 @@ def _legacy_check_initramfs_update_available(self,
         @return: True if a newer initramfs is available to download
         """
         branch = branch or self._default_branch
+        branch = "master" if branch == "stable" else branch
         if not self.initramfs_url:
             raise RuntimeError("No initramfs_url configured")
         initramfs_url = self.initramfs_url.format(branch)