buildextend-live: log a message when booting via the stub grub.cfg
At runtime it's immediately overwritten by the GRUB menu, but it shows up
in console logs.  This will help validate automated tests.
@@ -501,6 +501,7 @@ def generate_iso():
with open(os.path.join(tmpimageefidir, vendor_ids[0], "grub.cfg"), "w") as fh:
fh.write(f'''search --label "{volid}" --set root --no-floppy
set prefix=($root)/EFI/{vendor_ids[0]}
+echo "Booting via ESP..."
configfile $prefix/grub.cfg
boot
''')
