AT90USB1286 PIO cleanup and optimization (#11230)

-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.

buildroot/share/PlatformIO/boards/at90usb1286.json

@@ -1,14 +1,14 @@
 {
   "build": {
     "core": "teensy",
-    "extra_flags": "-DTEENSY2PP",
+    "extra_flags": "-DTEENSY2PP  -fsingle-precision-constant",
     "f_cpu": "16000000L",
     "mcu": "at90usb1286"
   },
   "frameworks": [
     "arduino"
   ],
-  "name": "at90USB1286.json",
+  "name": "Atmel AT90USB1286 based",
   "upload": {
     "maximum_ram_size": 8192,
     "maximum_size": 122880,