boot: re-establish RELEASE_MUTABLE_DIRECTORY, with cleaner semantics
boot: re-establish RELEASE_MUTABLE_DIRECTORY, with cleaner semantics

Bring back R_M_D as 'var', to place logs, pipes and
generated config files.

 - Simplifies symlink-to-dev-files support

 - Files in running-environment get a comment
   indicating "Generated, create/edit in path/to/release/foo

 - Does not ovewrite release or user-edited files.

 - Supports readonly releases (from the PoV of the process).

bitwalker/distillery,bitwalker/distillery