I'm sure you know that the Raspberry line of boards are quasi-Open Source; that is, the Broadcom low-level firmware is not (as far as I can determine) open: https://github.com/raspberrypi/firmwarefirmware
This repository contains pre-compiled binaries of the current Raspberry Pi kernel and modules, userspace libraries, and bootloader/GPU firmware.
Depending upon the spirit of the Open Source target you are shooting for, this simply could be a brick wall
When I said "Open Source", I should have phrased that differently.
I was interested in modifying the Android OS, and having the source code available was the important thing, but whether the whole thing was completely Open Source
With Android, as far as I can tell, no one produces hardware, including phones and tablets, which is completely open right down to the hardware drivers level.
(Actually, I guess it may be possible to build Android for the X86 architecture which may be totally open source, but I think it would be extremely difficult to do)
Since I last posted, I have managed to build LineageOS (Android 7.1) for my 3 year old Moto G2 phone, from the sources downloaded from github.
But, part of the build process, requires that some proprietary binaries are extracted from the phone, hence its not fully open source.
It took my Core i5 machine, nearly 5 hours to build the sources, but it worked in the end!