platformio.ini:

[env:disco_f407vg]
platform = ststm32
board = disco_f407vg
board_build.mcu = stm32f407vet6
framework = libopencm3
upload_protocol = stlink

Gives:

CURRENT: upload_protocol = stlink
Uploading .pio\build\disco_f407vg\firmware.elf
xPack OpenOCD, 64-bit Open On-Chip Debugger 0.10.0+dev (2019-07-17-11:28)
Licensed under GNU GPL v2
For bug reports, read
        http://openocd.org/doc/doxygen/bugs.html
debug_level: 1

srst_only separate srst_nogate srst_open_drain connect_deassert_srst

Error: timed out while waiting for target halted
embedded:startup.tcl:449: Error: ** Unable to reset target **
in procedure 'program'
in procedure 'program_error' called at file "embedded:startup.tcl", line 474
at file "embedded:startup.tcl", line 449
*** [upload] Error 1
==================================== [FAILED] Took 3.03 seconds ====================================