Don't append port details to avrdude for usbasp

When usbasp is used as programmer, there is no need to specify the port
to avrdude.

Fix #123
Ignore commented lines while parsing boards.txt file

Fix #124
In ISP mode, read baudrate and programmer from boards.txt

Arduino IDE also reads this from boards.txt file

Fix #125
Add `burn_bootloader` target

Code to burn fuses is moved from ispload target to this new target, so
that fuses are burned only once when needed.

Fix #85
Show correct path to `arduino.mk` file in help message

Fix #120