-normalize `env` and `board` to lowercase naming convention.
-make board `name` follow descriptive convention.
-implement `-fsingle-precision-constant` compile optimization per https://github.com/MarlinFirmware/Marlin/pull/11178#issuecomment-401673901
-fix typo in 5DPRINT entry.

This reverts commit a26fdf6b.

On a leap of faith...  I'm merging...

See https://github.com/MarlinFirmware/Marlin/pull/11079#issuecomment-399309709

-Enable boards_dir option in PIO INI
-Remove defunct custom board file operations for PIO auto-build environment
-Remove now-defunct Teensy++ 2.0 PIO entry, replaced by `at90USB1286_DFU` and `at90USB1286_DFU` (using this entry always put AT90USB board boot loaders in jeopardy of being overwritten due to the very small size of the official Teensy++ 2.0 bootloader)
-whitespace

- TMC2130Stepper v2.2.0
- TMC2208Stepper v0.0.4

Followup to #10096

- Compiler flags so ELF will include the original source.
- Use commit-id archives for a working `lib_ignore`.