Fixed a problem where mpide crashes at startup if the board in preferences is not found

If the board= entry in preferences.txt is not found in any boards.txt file mpide dies with a messy exception:

    Exception in thread "main" java.lang.NullPointerException
	at processing.app.Base.<init>(Base.java:273)
	at processing.app.Base.main(Base.java:183)

This update fixes the problem by defaulting the board to the first entry found in the Target.getBoards() map if the board isn't found.