The input format is now determined by the OUTPUT_EXT key in
targets.json, and defaults to "bin" when not specified. This changes the
Teensy3_1 and the NRF51x targets' post bulid hooks.
Teensy3_1 just converted to intelhex, so we do nothing instead.
NRF51x assumed that it was taking in a bin format file. I made it detect
file type by extension.