Now reads the RTC peripheral at the end of each received packet.
TODO: Improve resolution to milliseconds or better.
TODO: Work back from end of packet to compute timestamp for beginning of packet.
TODO: Reuse ChibiOS RTC code, which isn't used now because ChibiOS on M0 core is responsible for RTC configuration, and including ChibiOS RTC API on M4 will also try to initialize/manage the peripheral.
