HID report transmission partially works now

partially i.e. if you attach the keyboard before the noise handshake. I
suspect some memory corruption somewhere.

src/packet_interface.c

@@ -57,3 +57,9 @@ void usart2_isr(void) {
     } /* else just return and wait for next byte */
 }
 
+void send_packet(struct dma_usart_file *f, const uint8_t *data, size_t len) {
+    /* ignore return value as putf is blocking and always succeeds */
+    (void)cobs_encode_incremental(f, putf, (char *)data, len);
+    flush(f);
+}
+