Writes the string into a file. The filename is relative to the location that is specified (this is a flat directory).
If the file exists, it is overwritten with the string. If the file does not exist, it is created.
Read-only location parameters:
app The app's own directory (so you can read files included in the app's zip file).
Read and write location parameters:
nv Non-volatile storage. Data stored here survives firmware updates.
Name of the file to act on.
String to act on.
Using digium.readFile / digium.writeFile to store settings.