there aren't clear instructions right now to do something like that. The main reason is that we initially used some QCOM tools to generate the *.img files that are used by the installer. We are about to move to new toolset that doesn't use any proprietary tools. I have said a few times that I will be doing that soon.. but it keeps getting down the todo list.. I am really hoping to be do that in March (actually for some unrelated reasons I have to get that done in March..).
We started from the RPi Noobs , as you noticed, but it has been modified quite a bit. Noobs quite tied to the RPi with a bunch of 'hard coded' params, and the authors didn't really want to add support for other boards.
The git tree that you find is where I keep the current installer (which is more recent that the one currently in use in our images). The installer was recently ported from Qt4/Embedded to Qt5, and now runs on top of a minimal X11/Debian image.
The soon-to-be-released installer will consist of:
* debian minimal X11 image running the 4.4 kernel
* db-installer Qt5 application
The new installer will support flashing the regular fastboot images instead of the custom 'split' images that we have so far. That should allow anyone to create their own (custom) .img files and add them into the SD card.