makery.mk: Comment logical processing steps
gmsl: Update to GMSL v1.1.6
srcs: Rename SRCS_files_preq -> SRCS_preqs, it's a bit clearer
srcs-preprocess: Get rid of unused variables and rules
srcs-preprocess: Add functions to get info from upstream preprocessor modules
srcs-preprocess: Add readme documenting preprocessor module protocol
srcs-preprocess: Prefix special variable names in preprocessor modules with "pp"