-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.

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