Check returns false if too many failures are recorded for an image, true otherwise.

Reads history file, comparing stored count with MaxFailures.

When added to log.Preboot, RebootHook updates the history file at the end of factory restore.

RecordBootState records boot status (success/failure, how badly failed).

Sets where the history file can be found.

Writes arbitrary records to history file, for use in integ tests.

max allowed sum of ImagingFailures and BootFailures.