dtoverlay: Break on an error when parsing fragment overrides
dtoverlay: Break on an error when parsing fragment overrides

A misformed string in an override, eg
changeme = <0>, "-1,-2";
would stall the parsing as it didn't know how to handle ','
and kept trying to process it over and over.

Abort on incorrect syntax.

raspberrypi/userland,raspberrypi/userland,raspberrypi/userland