As pointed out at https://github.com/MarlinFirmware/Marlin/issues/7981#issuecomment-407116642

-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.

Indeed the HAL does not need to be mucked around with to expose ATmega2560 pins not available as numbers on the MEGA board, I'll need to update the wiki with that tidbit and a reference to the pin-mapping comment in `fastio_1280.h`.

See #11097

As noted in #10965

- Ordering and match classification between `boards.h` and `pins.h`
- Check `pins.h` environments

Co-Authored-By: ejtagle <ejtagle@hotmail.com>

Co-Authored-By: Simon Swenson <simon-swenson-8351@users.noreply.github.com>

Based on #10519

Co-Authored-By: Alex <alex18881@users.noreply.github.com>

What should the SD detect and SDSS pins be? Followup soon on #8138