Update libnfc to 1.7.0-rc6, add  USB CDC/ACM patch

Update libnfc to latest, add config file (generated by brew, in prefix)
and add a patch to support PN532-based NFC devices with USB CDC/ACM
interfaces, such as the OpenPCD2
(http://www.openbeacon.org/OpenPCD_2_RFID_Reader_for_13.56MHz)

(Formula simplified thanks to jacknagel).