I have completed the rpi-image-tool system.

The rpi-image-tool system is able to:

• Create a new and empty .img file containing the necessary small FAT32 boot partition and the larger Linux partition.
• Mount the two partitions from an existing .img file, for example one downloaded ffrom the Web.
• Mount the two partitions contained on an SD card plugged into the host machine.

Note that the code assumes two partitions:

1. FAT32 boot partition.
2. Linux root partition.

It assumes the partitions appear in that order.

See the documentation elsewhere on this site or grab the repository and install and read the help.

You can see the help with:

rpi-image-tool -h

Please pay particular attention to the important caveats at the bottom of the usage message.