syslinux: help git grep *_size

It's not exactly obvious how install2_size, live_size or rescue_size
get defined since the variable names themselves get constructed;
help git grep these down.

features.in/syslinux/scripts.d/20-propagator-ramdisk

@@ -1,6 +1,7 @@
 #!/bin/sh
 # postprocess isolinux configuration
 # when the image sizes are finally known
+# NB: install2_size, live_size, rescue_size get defined here
 
 cd "$WORKDIR"