It:s the Firefly sdk (which is basically a rockchip sdk, the same that gpd use, because it is what rockchip provide to them). But some files must be taken from stock ROM (framework.jar, Mali new drivers for those that use it(the called newmali version), or the gamepad.apk), because things like gpd mapper or newer Mali drivers are not in Firefly sdk. They are added by gpd in their firmwares. But the firmware itself is compiled using Firefly sdk except for those things I mentioned that I am forced to take them from stock ROM). Mali drivers are not even open for gpd, because they are close source by the own rockchip.
Have in mind companies generally only release kernel code (the ones who do), but the full sdk is rarely shared by companies. The case of Firefly board is special because is a development board, so they always share everything. They released a board with the new rk3399 which I am currently helping to test (and buggy as hell to tell the truth)
So to sum up, my ROM is based on Firefly sdk which is close to aosp (Nexus products sdk), and a couple of things that must be added from stock ROM because I am forced to do it, because they are not open source and are not in Firefly sdk, like gpd mapper, the APK to select Xbox/PS3 modes, etc)