Implement support for MFRC522 card reader in doorduino.

Add file tools/mfrc522.c, with code to check for the presence of a card, and
read an ID string from the card if present.

Move the existing RFID soft-serial interface to pins 8 and 9, to free up
hardware SPI pins for the MFRC522 communication.

Check the MFRC522 every 1/4 seconds, and inject any card data into the card
data buffer, flashing status LED to give visual indication of "card read".

Fix makefile to work with multiple object files.
Fix incorrect reference to interrupt pin (forgotten when moving softserial
from pin 12 to pin 8).

RFID1 tested to work now with the new code.