Move packet timestamping into baseband.

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.

firmware/common/ais_packet.cpp

@@ -128,8 +128,8 @@ bool Packet::is_valid() const {
 	return length_valid() && crc_ok();
 }
 
-rtc::RTC Packet::received_at() const {
-	return received_at_;
+Timestamp Packet::received_at() const {
+	return packet_.timestamp();
 }
 
 uint32_t Packet::message_id() const {