- Remove the two version of 16mHz. Crystal and resonator use the same
fuse setting. Also, Arduino IDE only showed the first one 16Mhz as a
board option.

- Added 20Mhz board option.

Symptoms for incorrect fuse setting:
- X axis move in one direction
- limit switches fail to trigger
- unable to reprogram the device via bootloader
- stops working when the reset switch is pressed.
- device fails to bootup when power by a slow raise power supply.

Solution:
- Disabled the jtag
- set the correct clock selection bit
- set the correct bootloader memory size
- enable brown-out setting
- enable clock power up delay