[docs] unable to flash STM32F4DISCO board by following README.md

[qiaojingx]

Description

Error thrown when flash STM32F4DISCO board by taking steps from ST STM32F4DISCOVERY Platform. I am wondering if a specific tools needed, like SWD debugger?

Steps to Reproduction

1. make BOARD=stm32f4_disco ide
2. Connect the Mini USB cable from the device(CN1) to your PC.
3. Connect a micro-USB cable from the device (CN5) to your PC.
4. Flash the application using the following command: make BOARD=stm32f4_disco flash

Actual Result

Open On-Chip Debugger 0.10.0-g7852ae77-dirty (2017-10-15-17:20)
Licensed under GNU GPL v2
For bug reports, read
	http://openocd.org/doc/doxygen/bugs.html
Info : The selected transport took over low-level target control. The results might differ compared to plain JTAG/SWD
adapter speed: 2000 kHz
adapter_nsrst_delay: 100
none separate
srst_only separate srst_nogate srst_open_drain connect_deassert_srst
Info : Unable to match requested speed 2000 kHz, using 1800 kHz
Info : Unable to match requested speed 2000 kHz, using 1800 kHz
Info : clock speed 1800 kHz
Error: open failed
in procedure 'init' 
in procedure 'ocd_bouncer'

Error: Command '['/opt/zephyr-sdk/sysroots/x86_64-pokysdk-linux/usr/bin/openocd', '-s', '/opt/zephyr-sdk/sysroots/x86_64-pokysdk-linux/usr/share/openocd/scripts', '-f', '/home/jo/zjs_test/weekly/zephyr.js/deps/zephyr/boards/arm/stm32f4_disco/support/openocd.cfg', '-c', 'init', '-c', 'targets', '-c', 'reset halt', '-c', 'flash write_image erase /home/jo/zjs_test/weekly/zephyr.js/outdir/stm32f4_disco/zephyr/zephyr.bin 0x8000000', '-c', 'reset halt', '-c', 'verify_image            /home/jo/zjs_test/weekly/zephyr.js/outdir/stm32f4_disco/zephyr/zephyr.bin 0x8000000', '-c', 'reset run', '-c', 'shutdown']' returned non-zero exit status 1
Re-run with VERBOSE=1 for a stack trace.
zephyr/cmake/flash/CMakeFiles/flash.dir/build.make:57: recipe for target 'zephyr/cmake/flash/CMakeFiles/flash' failed
make[4]: *** [zephyr/cmake/flash/CMakeFiles/flash] Error 1
make[4]: Leaving directory '/home/jo/zjs_test/weekly/zephyr.js/outdir/stm32f4_disco'
CMakeFiles/Makefile2:2191: recipe for target 'zephyr/cmake/flash/CMakeFiles/flash.dir/all' failed
make[3]: *** [zephyr/cmake/flash/CMakeFiles/flash.dir/all] Error 2
make[3]: Leaving directory '/home/jo/zjs_test/weekly/zephyr.js/outdir/stm32f4_disco'
CMakeFiles/Makefile2:2198: recipe for target 'zephyr/cmake/flash/CMakeFiles/flash.dir/rule' failed
make[2]: *** [zephyr/cmake/flash/CMakeFiles/flash.dir/rule] Error 2
make[2]: Leaving directory '/home/jo/zjs_test/weekly/zephyr.js/outdir/stm32f4_disco'
Makefile:430: recipe for target 'flash' failed
make[1]: *** [flash] Error 2
make[1]: Leaving directory '/home/jo/zjs_test/weekly/zephyr.js/outdir/stm32f4_disco'
Makefile:204: recipe for target 'flash' failed
make: *** [flash] Error 2

Expected Result

1. If specific tools needed, please add detailed information to README.md.
2. If no other tools needed, please help to fix this issue.

Test Builds

Branch	Commit Id	Target Device	Test Date	Result
master	9db8f2b	STM32F4DISCO	Mar.09, 2018	Fail

Additional Information

We test pass two ways to flash STM32F4DISCO board now.

1. Copy .bin to device like FRDM-K64F.
   cp outdir/stm32f4_disco/zephyr/zephyr.bin /media/<username>/DIS_F407VG/
2. flash with STLINK tools.
   st-flash write outdir/stm32f4_disco/zephyr/zephyr.bin 0x8000000