Hey, when I just built my own Firefox OS (b2g) image for my Firefox OS Tablet (flatfish) straight from the latest sources, I ran into this (known) problem:

Can't find necessary file(s) of Bluedroid in the backup-flatfish folder.
Please update the system image for supporting Bluedroid (Bug-986314),
so that the needed binary files can be extracted from your flatfish device.

So, as I struggled to figure out the exact instructions on how to proceed from this, I figured I should jot down what I did in the hopes that it perhaps will help a fellow hacker at some point:

1. Download the 3 *.img files from the dropbox site that is referenced from bug 986314.
2. Download the flash-flatfish.sh script from the same dropbox place
3. Make sure you have ‘fastboot’ installed (I’m mentioning this here because it turned out I didn’t and yet I have already built and flashed my Flame phone successfully without having it). “apt-get install android-tools-fastboot” solved it for me. Note that if it isn’t installed, the flash-flatfish.sh script will claim that the device is not in fastboot mode and stop with an error message saying so.
4. Finally: run the script “./flash-flatfish.sh [dir with the 3 .img files]”
5. Once it has succeeded, the tablet reboots
6. Remove the backup-flatfish directory in the build dir.
7. Restart the flatfish build again and now it should get passed that Bluedroid nit

Enjoy!