DOC: STM32F4 add example flash/reset/debug information.

* Information on how to flash NuttX firmware to STM32F4 target was missing
  in the documentation and asked by new users on the dev@ list.
* Example on how to Flash, Reset, Debug with OpenOCD is now added to docs.

Signed-off-by: Tomasz 'CeDeROM' CEDRO <tomek@cedro.info>

Documentation/platforms/arm/stm32f4/index.rst

@@ -519,6 +519,33 @@ Protected Mode Build
      If you do this a lot, you will probably want to invest a little time
      to develop a tool to automate these steps.
 
+Flashing and Debugging
+======================
+
+NuttX firmware Flashing with STLink probe and OpenOCD::
+
+   openocd -f  interface/stlink.cfg -f target/stm32f4x.cfg -c 'program nuttx.bin 0x08000000; reset run; exit'
+
+Remote target Reset with STLink probe and OpenOCD::
+
+   openocd -f interface/stlink.cfg -f target/stm32f4x.cfg -c 'init; reset run; exit'
+
+Remote target Debug with STLink probe and OpenOCD:
+
+ 1. You need to have NuttX built with debug symbols, see :ref:`debugging`.
+
+ 2. Launch the OpenOCD GDB server::
+
+     openocd -f interface/stlink.cfg -f target/stm32f4x.cfg -c 'init; reset halt'
+
+ 3. You can now attach to remote OpenOCD GDB server with your favorite debugger,
+    for instance gdb::
+
+     arm-none-eabi-gdb --tui nuttx -ex 'target extended-remote localhost:3333'
+     (gdb) monitor reset halt
+     (gdb) breakpoint nsh_main
+     (gdb) continue
+
 Supported Boards
 ================