Set up logging fd before any flash-image code runs
Move all runtime data into dedicated per-run directory
Add large temporary directory support

The LARGE_TEMP_DIR environment variable may contain a path to a disk-
backed temporary storage location. The mktemp utility is now trans-
parently wrapped to add a new -l option which requests that the file
returned be contained in the large temporary directory if possible.
Make sure the mod_options file exists before grepping it