Allow base path overrides when running a recipe.

Allow base path overrides when running a recipe.

This is to allow running recipes via isolate, where we can't assume the normal
infra directory layout.

BUG=437264
R=iannucci@chromium.org,martiniss@chromium.org

[luqui, 2016-01-16 05:40:09]

ptal

I am not sure if this is a good solution.  First of all, I'm not sure it will
work at all -- will the build directory be available when running the isolate? 
I think there's some variable thing we can do.   

But also, for packages which use build as a dependency, build's stuff is going
to be in .recipe_deps, and it seems weird to say base_path_override={'build':
'.../.recipe_deps/build'}; we're breaking an abstraction boundary.

I think I would rather have these as resources a la package_resource... but
unfortunately some things needed don't live under scripts/slave so it doesn't
fit the current design.  Maybe that needs to be adjusted to support .. or be
relative to the repo root.  Eeeahh..

Thoughts?

[Paweł Hajdan Jr., 2016-01-19 21:32:14]

Description was changed from

==========
Allow base path overrides when running a recipe.

This is to allow running recipes via isolate, where we can't assume the normal
infra directory layout.

BUG=
R=iannucci@chromium.org,martiniss@chromium.org
==========

to

==========
Allow base path overrides when running a recipe.

This is to allow running recipes via isolate, where we can't assume the normal
infra directory layout.

BUG=437264
R=iannucci@chromium.org,martiniss@chromium.org
==========

[martiniss, 2016-01-19 23:29:38]

What if we make the recipe_engine do sym links into the recipe_deps? Hide the
weird hacky ness?

https://codereview.chromium.org/1592973003/diff/1/recipe_modules/path/config.py
File recipe_modules/path/config.py (right):

https://codereview.chromium.org/1592973003/diff/1/recipe_modules/path/config....
recipe_modules/path/config.py:54: c.base_paths.setdefault('slave_build', (
This won't work as is.... base_paths already has a value for slave_build, so
this won't change it. Should just assign here.