I reach the Sony Open Devices program page and found the last open source firmware package:
In particular this one:
Which contains two folder with similar content:
Someone can explain me the difference between the two? Which is the best to try?
Only source code apart these two images:
but nothing about binary firmware useful for an upgrade of the modem/
At this point, I found enough information to compile a new kernel and its booting image. Does anyone outside Jolla developer circle did a custom boot image before? Because creating a brand-new boot image is the first step to upgrade the kernel.