scripts: Allow specifying custom teensy-cores, don't check for teensy flasher.
scripts: Allow specifying custom teensy-cores, don't check for teensy flasher.

fixator/aprinter,ambrop72/aprinter,johnkingsley/aprinter,ambrop72/aprinter,fixator/aprinter,ambrop72/aprinter,ambrop72/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,fixator/aprinter,fixator/aprinter,fixator/aprinter,ambrop72/aprinter,johnkingsley/aprinter,ambrop72/aprinter,ambrop72/aprinter,johnkingsley/aprinter,fixator/aprinter