Introduce a RFM69 compatibility mode for RFM12.

    To enable the RFM69 and RFM12 to talk to each other, a few things in
    the RFM12 code need to be changed. These changes call into three
    buckets:

    1. Modify the payload structure to match that HW version in RFM69
    2. Modity the checksum algorithm to match the HW version in RFM69
    3. Tweak the radio parameters so they're compatible.

    The compatibility mode is enabled by

            #define RF69_COMPAT 1