Fix M503C

[GMagician]

This exhume #25687 closed because of a misunderstaning

[ellensp]

Confirmed working on avr.

[GMagician]

Continuing from close PR: @ellensp please note the new for cycle, this is the addition to your suggested/premature solution. it's this that let it works on AVR and prevent reboot on STM32. Maybe you missed it and this probably is the "misunderstanding" cause

[ellensp]

care to tack a bit on to this pr??

Another small part of issue is that Marlin encodes full path of marlin_config.json into MC.ZIP, leaking users user name and directory information.

Here is a patch for that. now only relative path from .pio/build onward is stored

diff --git a/buildroot/share/PlatformIO/scripts/signature.py b/buildroot/share/PlatformIO/scripts/signature.py
index 4fc0084e57..0da02f837b 100644
--- a/buildroot/share/PlatformIO/scripts/signature.py
+++ b/buildroot/share/PlatformIO/scripts/signature.py
@@ -56,13 +56,14 @@ def compute_build_signature(env):
     files_to_keep = [ 'Marlin/Configuration.h', 'Marlin/Configuration_adv.h' ]
 
     build_path = Path(env['PROJECT_BUILD_DIR'], env['PIOENV'])
+    build_path_relative = Path('.pio', 'build', env['PIOENV'])
 
     # Check if we can skip processing
     hashes = ''
     for header in files_to_keep:
         hashes += get_file_sha256sum(header)[0:10]
 
-    marlin_json = build_path / 'marlin_config.json'
+    marlin_json = build_path_relative / 'marlin_config.json'
     marlin_zip = build_path / 'mc.zip'
 
     # Read existing config file