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

Update libnfc to latest version, 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). This also removes the "with-pn532_uart" option as this is now handled automatically. Similarly, the "--enable-serial-autoprobe" option to libnfc has been deprecated in favour of an option in the config file.