Move some external files to external/ folder (#744)

It is lot easier to work with automatic formatters if we have external
files in different folder. For some tools we can example just exclude
external all together.

Co-authored-by: Kari Argillander <kari.argillander@fidelix.com>

ports/bdk-atxx4-mstp/external/bootloader/serial.h

@@ -0,0 +1,25 @@
+/*****************************************************************************
+*
+* Atmel Corporation
+*
+* File              : serial.h
+* Compiler          : IAR C 3.10C Kickstart, AVR-GCC/avr-libc(>= 1.2.5)
+* Revision          : $Revision: 1.7 $
+* Date              : $Date: Tuesday, June 07, 200 $
+* Updated by        : $Author: raapeland $
+*
+* Support mail      : avr@atmel.com
+*
+* Target platform   : All AVRs with bootloader support
+*
+* AppNote           : AVR109 - Self-programming
+*
+* Description       : Header file for serial.c
+****************************************************************************/
+
+void initbootuart(
+    void);
+void sendchar(
+    unsigned char);
+unsigned char recchar(
+    void);