FEATURE: Add RW support for USB disk images (USB_MOUNT_DISK_READ_WRITE)

[i-am-shodan]

Currently only USB_MOUNT_DISK_READ_ONLY is available to mount a USB disk image from the SD card filesystem. If you want any type of write access the only option is to use ATTACKMODE STORAGE to mount the entire SD card.

We'd like to add USB_MOUNT_DISK_READ_WRITE but the underlying Arduino FS libraries don't support this. One option would be to leave some unpartitioned space at the end of the drive and use that.