Add a way to declare the default keymap

[crawfxrd]

Replace hard-coding default.c with a KEYMAP variable or something that defines the default layout to use.

System76 laptop keyboards are changing their layouts starting with RPL. They use the same matrix layout as 15in_102, but may have different key layouts. For example, "Display Toggle" being replaced by "Mic Mute" on Fn layer of F2.

Ref: src/keyboard/system76/15in_102/keyboard.mk
Ref: system76/ec#366
Ref: system76/ec#352

[crawfxrd]

Alternatively, keyboards in System76 EC gets renamed (stop using "size and number of keys" as unique names) so every potential keyboard we can ever have does have a default.c. Then keyboard-configurator would only need to update the keyboard used.

[ids1024]

default.json is already per-laptop rather than per-"keyboard" in the Configurator, so it wouldn't be hard to update layouts.py to accommodate this.