Downloading Ubuntu images

This is a Makefile that automates the process of downloading Ubuntu ISO images.

Usage:

1. Edit the Makefile to pick the images you want to have and the Ubuntu mirror you want to use
2. Run make to download them
3. Run make verify-all to check the file checksums and GPG signatures

There are also Makefile targets to download or verify single files. Run make help to learn about them.

GPG signature checking assumes you're running on an Ubuntu system, which has the Ubuntu archive public keys in /usr/share/keyrings/ubuntu-archive-keyring.gpg