- Added FILAMENT_RUNOUT_DISTANCE_MM option
- Added FILAMENT_MOTION_SENSOR option

Co-Authored-By: Giuliano Zaro <gmagician@users.noreply.github.com>

Co-Authored-By: timoxd7 <timoxd7@users.noreply.github.com>

-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.

Also fix use_example_configs

Co-Authored-By: J.C. Nelson <xc0000005@users.noreply.github.com>

-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

130048 is available on genuine Teensy2.0++ with Paul's "Halfkay" bootloader.  Atmel DFU-equiped boards have only 122880 available