rpi_gen_config from scripts/mkimg.arm.sh and setup-disk should be merged
alpine-conf@c2f6f7d9 copied rpi_gen_config from scripts/mkimg.arm.sh into setup-disk. Ideally there should only be one version of this function. This would avoid duplicate changes (see !29263 (merged) and alpine-conf!51 (merged)) and make errors easier to spot (see alpine-conf!52 (closed)). Is there any precedent for sharing code between alpine-conf and mkimage?