rpi: Flush serial on cleanup

* Wait for transmit fifo to empty on serial cleanup
* Serial cleanup on platform exit

src/system/boot/platform/raspberrypi_arm/serial.cpp

@@ -67,7 +67,9 @@ serial_enable(void)
 extern "C" void
 serial_cleanup(void)
 {
-#warning IMPLEMENT serial_cleanup
+	// Transmit the last of our fifo
+	gLoaderUART.FlushTx();
+	gLoaderUART.Disable();
 }
 
 

src/system/boot/platform/raspberrypi_arm/start.c

@@ -92,7 +92,7 @@ platform_start_kernel(void)
 void
 platform_exit(void)
 {
-	#warning IMPLEMENT platform_exit
+	serial_cleanup();
 }