playing with some raspberry Pi, i came to build a stage4 image (that i called genBerry) and some installation scripts for it. Those scripts will take care of all the part of the installation: sdcard partitioning, formatting and mounting, kernel building and new portage tree on a squashfs image. You can, of course, customize the installation with some options when you call the scripts.
The compress image size is approx 190MB. I had tested it against raspberry 1A, 1B, zero and 2B. All the archive retrieved during the installation are verified against the public key of their author.
Once installed, the system contains no swap, /tmp and /var/log are tmpfs FS.
genBerry is built on the official stage3 for raspberryPi.
I'll be more than happy and grateful if some are interested in testing / using it and make feedback.
The link is here: https://github.com/dervishe-/genBerry