Add several features used for the reference runtime

This adds various functionality that I've been adding over the past few days for the new reference runtime. The gist of it is:

• Runtimes can extend each other (used so the HMI runtime can extend the headless one).
• pre_apt_script is available in an SDK-specific form (used to make sure development repos never end up in the platform variant).
• --override-package can be used to override specific packages to custom deb files (this is mostly just useful for testing purposes).
• Locales can be extracted into their own extension.
• Bundles now end in .flatpak, to make them easier to install.
• Custom overlays can be added on top of the runtime's filesystem (used to add custom fontconfig files).
• The font cache can be preserved after building.