bundles/raspberrypi: do not set empty default for config.txt

bundles/raspberrypi/files/config.txt

@@ -11,6 +11,6 @@ force_turbo=1
 arm_64bit=1
 % endif
 
-% for item in sorted(node.metadata['raspberrypi']['config.txt']):
+% for item in sorted(node.metadata['raspberrypi'].get('config.txt', set())):
 ${item}
 % endfor

bundles/raspberrypi/metadata.py

@@ -18,6 +18,5 @@ defaults = {
             'quiet',
             'plymouth.ignore-serial-consoles',
         },
-        'config.txt': {},
     },
 }