Have you considered this?
Have you tried the Yocto Project (TM)?
The Yocto Project is a registered trademark of the Linux Foundation.
With Yocto, you can create distros for Beaglebone and Pi, simply. You can use recipes to spice up the distro with your “flavor”!
Of course, you can always just create an image and dd…
But where’s the fun in that?
Check out Yocto if you are interested!
Github link to some recipes coming soooooooooon!