platforms in sketchbook should be additional to IDE's platforms, not remove any

consider there are 15 boards in 
	ide/hardware/arduino/avr/boards/

and you add 3 new boards in 
	sketchbook/hardware/arduino/avr/boards/

In earlier implementation this would have resulted in only the 3 new boards
available, and older 15 being removed.

This patch corrects it, and shows all 18 boards, (like it should ideally be)

Signed-off-by: Arnav Gupta <championswimmer@gmail.com>