[arch-releng] [archiso] Refactor configs/releng -> build only dual-iso on x86_64

Hello I want to know is this idea is still valid/wanted, was discussed when [core] generation was removed, and distribute only dual-iso. Pierre talks about removing single iso generation also from build.sh script (if I do not remember bad). So... My plan is: (1) Remove posibility of building single-iso. (2) Build only dual-iso. (3) Per (1) and (2) => configs/releng/build.sh will only work on x86_64. Some time ago I made some modifications, but I lost the changes, I remember that as result, the code was reduced but I have some issues and I did not finished at all. Feedback is really apreciated. -- Gerardo Exequiel Pozzi \cos^2\alpha + \sin^2\alpha = 1