Enable local board repository for PIO (#11079)

-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