Add the ability to modify base.toml for freezedry configuration

[agajews]

@Limero 's issue:

As an advance user, it would be great to get the option to modify what packages from base.toml are included in a freezedry configuration. That way, you could disable packages like printer drivers and Wi-Fi related things if you wanted to slim down your install.

Another option would be to add the ability to completely ignore base.toml and let the user manually specify all wanted things in the regular packages section on a freezedry configuration.

[agajews]

I'm moving this to the freezedry repo. The simplest solution would be to make a minimal core.toml config that contains base, base-devel, freezedry, and freezedry's immediate dependencies.