bsod: Write crashlog to flash or tmp when there's no /media/hdd

If the file cannot be written to /media/hdd, place it in /home/root
or in /tmp. Do not use a timestamped filename in that case, and do
not overwrite an existing crashlog in /home/root to prevent damaging
the flash. Use /tmp if /home/root isn't available.

Also display the log filename and fix a GCC warning.