Switch FAT partition to read-only and make look like a CD

[jadonk]

Do we really need people to edit uEnv.txt from the shared drive these days? Perhaps, but if Linux is booted it is rare that you can't at least open the virtual serial console and edit from the Linux side. I'm concerned about people corrupting it and it would allow autorun.inf to work such that Windows users start to discover the START.htm.

[jadonk]

@RobertCNelson if you don't object, I'll make the change to image-builder, as I think most people should be able to get into enough of Linux to edit this file.

[RobertCNelson]

Hum, I don't even mount it by default with /etc/fstab anymore:
https://github.com/RobertCNelson/omap-image-builder/blob/master/tools/setup_sdcard.sh#L942

So it should be save to wrap it in something like, after above:

if [ "x${bborg_production}" = "xenable" ] ; then
        echo "" >> ${wfile}
fi