Allow custom checkout root variables

Pigweed sets PW_ROOT, but dependent projects may need to set their own
variables pointing to the checkout root. Add option so builders can
specify the name of that top-level variable (e.g., 'FOO_ROOT').

Change-Id: If702778b4196581dc7cc3aa1432bfcc86fc971ed
5 files changed
tree: 087da05a112dfacafcf8683413f1a365e02c3ad9
  1. infra/
  2. recipe_modules/
  3. recipes/
  4. .gitignore
  5. .style.yapf
  6. recipes.py