Add preset support

Add preset support

Presets can be defined in mkosi.presets/. A preset is just like a
regular config file/directory, except that mkosi can build multiple
presets sequentially.

If mkosi.presets/ exists, for each preset mkosi will read the global
configuration, followed by the individual preset configuration. It
will then build each of the presets in alpha-numerical order. Later
presets can use outputs of earlier presets, specifically using the
BaseTrees= and Initrds= options.

While this has many use cases, one promising use case is to allow
building an initrd and a final image that uses that initrd within a
single invocation of mkosi.