Create and enable a swapfile for devices with USB support. It is assumed that extra USB storage is already configured via opkg and fstab, as the swapfile most likely won't fit into the MTD storage.
The variable $SWPATH defines where the swapfile will be located in the filesystem, so it must be changed accordingly before the script is executed. Default value is `/overlay/swapfile`
- create: if there is enough storage space in the device, and no swap memory already in use, it creates a swapfile which has the same size as the device's RAM and enables it to run at boot time;