dir field for segments

[AngheloAlf]

Allow specifying a dir that would be prefixed to every file of the segment when emitting the linker script.

The following example should emit build/src/battle/area/omo2_1/actor/shy_squad.o as the file path.

settings:
  base_path: build

segments:
  - name: omo2_1
    dir: src/battle/area/omo2_1
    files:
      - { path: actor/shy_squad.o }