This adds support for having a custom install/deploy, where if `/etc/os-installer/deploy.sh` exists then pmbootstrap generates an image, exports it, then calls deploy.sh with the path to the image it created, and does NOT call pmbootstrap install with --disk. deploy.sh comes from device packages that need to implement custom image deployment stuff, when using --disk is not appropriate. If deploy.sh doesn't exist then the standard install (using --disk) will take place. Signed-off-by: Clayton Craft <craftyguy@postmarketos.org> Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7112>