Answer
After booting the latest standard 60-som image from SDcard you can use "fw_update" to write the swu file to the 60-soms NAND memory.
Usage:
fw_update -m complete <path to swu-file>
<path to swu file> can be to a USB drive, a networl location, a swu-file on the SDcard, etc.
E.g. with a USB Stick mounted at /media and the som60.swu file present on the USB stick top directory:
fw_update -m complete /media/som60.swu