Enable LED3 and LED4 for the A3BU_XPLAINED board

LED3 is the red LED in the status LED
LED4 is the green LED in the status LED

They're arbitrary, but this order makes the
USB_ERROR and USB_READY masks in most of the demos light
the red for error and green for ready, which seems preferable.

I noticed that there is another pull request that you mentioned you couldn't do anything with because the github is a svn mirror.  Is there a better way to submit patches? :smile: