Macbook 7.1 (mid-2010): upgrade from 10.8 to 10.12
Two solutions were kindly given by Allan, I chose the "easiest" one: 


In recovery mode, access to Terminal
In /Volumes/"Macintosh HD"/System/Library/Extensions directory, remove Huawei .kext files (using rm -rf command) or other non-essential kext files. It appears that, for some reason, many users had issues with Huawei kext files...
After reboot, the installation was completed seamlessly, Sierra is finally installed.