Currently, the only way to define keyboard layouts is using KLL, the Keyboard Layout Language.
KLL outlines a complex keymapping scheme for custom keyboard firmware. It is based upon the concept of trigger:result pairs and ways to configure both parts of the pair (input and output). A variety of macros are supported along with analog keyswitches. In addition, this keymapping scheme supports static and dynamic layering as well as supporting arbitrary keyboard features that may or may not be available on a given keyboard.
For full information about the spec please read the linked pdf above.
TODO: Add some examples for how to do simple kll things such as adding a new layout (workman?) or editing/adding layers.
NOTE: Each keymap is done after the entire file is processed. This means that within the file the order of assignment does not matter (if you assign the same thing twice, then yes the most recent one takes priority).
NOTE: Don't use defaultMap.kll to change your layouts. This will work, but they will not be portable.
For information on how to use and combine kll files please see the Keymaps page.