base: Add flash-kernel and mtd-tools to the dependency list
As suggested by Guido, adding them here initially is better than in librem5-base, because only the devkit option in image-builder makes use of librem5-devkit-base, and therefore there's no risk of runing other builds.
WIP because the correct version will only (hopefully) get selected after flash-kernel!5 (merged) is in and in the repos.
Even then I'm not sure how to test it with image-builder before pushing to the CI repo.
Edited by Dorota Czaplejewicz