Document built-in variables in deployment template configuration

[stewartadam]

Both MODULES.foo and ${MODULEDIR<../relative-path>} appear to be supported and replaced by iotedgedev genconfig but neither pattern is documented.

It is unclear:

1. What the value of $MODULEDIR?
2. PR #391 also appear to show it's customizable with MODULES_PATH which is undocumented
3. What is required to have foo recognized in MODULES.foo?
4. Which mechanism should be used / is preferred?

More broadly - why are there two ways to access modules, and why is the $MODULEDIR treated as a pseudo-variable with different syntax from all the other environment variables? The non-standard syntax for path delcarations is confusing at best.

If it's attempting to construct a path to a module, why not just let the user construct it themselves? ${MODULEDIR}/../relative-path would maintain consistency with all other variables used in the templates and follow standard convention, instead of requiring something specific to IoT Edge.

[marianan]

thanks @stewartadam for sharing your questions. Our team inherited the tool last year and don't have any insights into these decisions. We will investigate and follow back here to clarify and document.