Main reqa

The main_reqa firmware is a proof-of-concept to run [parts of] the RFID stack inside the AT91SAM7 of OpenPCD.

main_reqa supports:
 * repetitive (endless loop) transmission of ISO 14443-A REQA packets
 * this is the default mode after power up
 * can be explicitly selected by pressing 'r' key on DBGU port
 * repetitive (endless loop) transmission of ISO 14443-A WUPA packets
 * can be selected by pressing 'w' key on DBGU port
 * repetitive full ISO 14443-A anticollision loop and SELECT
 * can be selected by pressing 'a' key on DBGU port

The response or error (if any) from the transponder/card will be displayed on the DBGU port, too.

main_reqa will most likely be a starting point for a full librfid-on-SAM7 implementation later on.