Add lock file support

[jjcarstens]

Running fwup writes while another fwup process is writing the same drive could be detrimental. Adding a lock file that can be used to infer another fwup process is running could be useful for cases where this needs to strictly be avoided.

Based on offline conversation with Frank, the initial suggestion for implementation is:

1. Figure out the lock file path. Maybe /tmp/fwup.mmcblk0 or something like that.
2. Create the lock file if it hasn’t already been created
3. Register an advisory lock on the file (via lockf). If this fails, then someone else is fwuping
4. Register an at_exit to erase the file