Expand make variables in write_file content and expand_template substitutions

If a user wants make variable substitution, they need to either write their own versions of these rules or else fall back to genrule.

I assume a boolean attribute would need to be added to toggle this behavior as it wouldn't be backwards-compatible.

Start by locating the write_file rule and the expand_template substitutions described in the issue, then read their current handling of content and substitutions. Determine how make-variable expansion should be enabled without changing existing behavior; done means both requested locations support the opt-in behavior and the backward-compatibility choice is documented by the implementation or tests.

Written by the indexing model from the issue text.