Improve file datestamping

[IanSB]

At the moment all files written on the SD card have the same date & time: 9/4/2016 01:00 It might be useful to at least make it the build date/time of the current firmware which would give a rough timeline for any saved profiles etc. This could be taken further by enumerating all files on the SD card and using the most recent date/time as that would push the time forwards if any file had been manually edited/created on a PC.