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/baseband/packet_builder.hpp

@@ -89,6 +89,7 @@ public:
 			}
 
 			if( end(bit_history, packet.size()) ) {
+				packet.set_timestamp(Timestamp::now());
 				payload_handler(packet);
 				reset_state();
 			} else {