Config file: Add mechanism for avoiding duplication between board & subsystem plugins

Only a few lines from the config file might differ between the board & subsystem plugins.

Example from the GT plugin:

hardwareType: serenity

libraries:
  - libgt_swatch.so

service:
  id: artix
  class: serenity::swatch::Artix
  uri: ipbuspcie-2.0:///dev/serenity_pcie/artix/h2c,/dev/serenity_pcie/artix/c2h
  addressTable: file:///opt/smash/etc/serenity/base/uHAL/v0.2/top_artix.xml

processors:
  - id: x0
    class: gt::swatch::FinorProcessor
    uri: ipbuspcie-2.0:///dev/serenity_pcie/x0/h2c,/dev/serenity_pcie/x0/c2h
  - id: x1
    class: gt::swatch::FinorProcessor
    uri: ipbuspcie-2.0:///dev/serenity_pcie/x1/h2c,/dev/serenity_pcie/x1/c2h

frontPanel: /usr/etc/serenity-swatch/frontpanel.txt

Possible solution: extends / include field, that references the base config for the board, and allows subsystems to override some fields.