Add Stm32F7xxdisco board support files

Status with examples/device/cdc_msc:
- make BOARD=stm32f723disco        => OK
- make BOARD=stm32f723disco PORT=1 => No Reaction
- make BOARD=stm32f746disco        => OK
- make BOARD=stm32f746disco PORT=1 => Hangs during enumeration
- make BOARD=stm32f769disco        => Hangs during enumeration

examples/readme.md

@@ -37,6 +37,10 @@ To compile for debugging with debug symbols add DEBUG=1, for example
 
 ```
 $ make BOARD=feather_nrf52840_express DEBUG=1 all
+If a board has several ports, one port is choosen by default in the individual board.mk file. Choose another port with PORT=x. For example to select the HS port of a STM32F746Disco board, use:
+
+```
+$ make BOARD=stm32f746disco PORT=1 all
 ```
 
 ### Debug Log