- previously we just removed the directory contents and not the directory itself

- happens if building with STRIPPED_LIBGCC

- allow overriding PROJECT_NAME (useful if doing CI builds / docker and the mounted directory name is generic)
- new target 'zip_release' (more human-friendly alias for $(BIN_TARGET))
- add new target 'build_dir' which copies stuff going into the release zip into a dir named build/ first (easier to inspect/zip)
- split out BUILD_DATE var for easier access
- for release/zip builds, create a version.txt with the makefile version#'s/date in it
- cleanup Makefile documentation a bit
- lint: fix syntax warnings with single quotes in "if you are on a '66 reconfigure CHIP"
- zip command always overwrites the file completely instead of appeding
- fix various issues with clean and purge not quite getting everything

- expose in version strings so we know if this build came from a Continous Integration build system (like Github Actions / Jenkins/etc)
- by default it's set to zero, and it's expected CI systems will set this to 1 from their command line