The only change from Arduino AVR Boards 1.6.20 to 1.6.21 was the change of the Arduino Nano board's Tools > Processor > ATmega328P menu item to use the Optiboot bootloader and the addition of the Tools > Processor > ATmega328P (Old bootloader) menu option to retain support for Nanos which use the classic bootloader. Other than that the two versions are identical. The reason for this change was that new Arduino Nanos are now being shipped with the Optiboot bootloader.