boards/stm32f746g-disco: use connect_assert_srst with openocd

[aabadie]

Contribution description

This PR enables connect_assert_srst in the reset_config option of OpenOCD for the stm32f746g-disco board. Without this option, the board is sometimes unflashable automatically (especially just after it's plugged to the computer).

Testing procedure

• on master I sometimes get the following openocd output:

$ BOARD=stm32f746g-disco make -C examples/hello-world flash-only --no-print-directory 
/work/riot/RIOT/dist/tools/openocd/openocd.sh flash /work/riot/RIOT/examples/hello-world/bin/stm32f746g-disco/hello-world.elf
### Flashing Target ###
Open On-Chip Debugger 0.11.0+dev-00546-g5795f4d3e (2021-12-23-10:48)
Licensed under GNU GPL v2
For bug reports, read
	http://openocd.org/doc/doxygen/bugs.html
hla_swd
Info : The selected transport took over low-level target control. The results might differ compared to plain JTAG/SWD
Info : DEPRECATED target event trace-config; use TPIU events {pre,post}-{enable,disable}
Info : clock speed 2000 kHz
Info : STLINK V2J25M14 (API v2) VID:PID 0483:374B
Info : Target voltage: 3.224115
Error: Cortex-M PARTNO 0x0 is unrecognized
Warn : target stm32f7x.cpu examination failed
Info : starting gdb server for stm32f7x.cpu on 0
Info : Listening on port 45791 for gdb connections
    TargetName         Type       Endian TapName            State       
--  ------------------ ---------- ------ ------------------ ------------
 0* stm32f7x.cpu       hla_target little stm32f7x.cpu       unknown

Info : Unable to match requested speed 2000 kHz, using 1800 kHz
Info : Unable to match requested speed 2000 kHz, using 1800 kHz
Error: Cortex-M PARTNO 0x0 is unrecognized
TARGET: stm32f7x.cpu - Not halted
make: *** [/work/riot/RIOT/examples/hello-world/../../Makefile.include:807: flash-only] Error 1

The second time it works though:

$ BOARD=stm32f746g-disco make -C examples/hello-world flash-only --no-print-directory 
/work/riot/RIOT/dist/tools/openocd/openocd.sh flash /work/riot/RIOT/examples/hello-world/bin/stm32f746g-disco/hello-world.elf
### Flashing Target ###
Open On-Chip Debugger 0.11.0+dev-00546-g5795f4d3e (2021-12-23-10:48)
Licensed under GNU GPL v2
For bug reports, read
	http://openocd.org/doc/doxygen/bugs.html
hla_swd
Info : The selected transport took over low-level target control. The results might differ compared to plain JTAG/SWD
Info : DEPRECATED target event trace-config; use TPIU events {pre,post}-{enable,disable}
Info : clock speed 2000 kHz
Info : STLINK V2J25M14 (API v2) VID:PID 0483:374B
Info : Target voltage: 3.227261
Info : stm32f7x.cpu: Cortex-M7 r0p1 processor detected
Warn : Silicon bug: single stepping may enter pending exception handler!
Info : stm32f7x.cpu: target has 8 breakpoints, 4 watchpoints
Info : starting gdb server for stm32f7x.cpu on 0
Info : Listening on port 34797 for gdb connections
    TargetName         Type       Endian TapName            State       
--  ------------------ ---------- ------ ------------------ ------------
 0* stm32f7x.cpu       hla_target little stm32f7x.cpu       halted

Info : Unable to match requested speed 2000 kHz, using 1800 kHz
Info : Unable to match requested speed 2000 kHz, using 1800 kHz
target halted due to debug-request, current mode: Thread 
xPSR: 0x01000000 pc: 0x08000918 msp: 0x20000200
Info : device id = 0x10016449
Info : flash size = 1024 kbytes
auto erase enabled
wrote 32768 bytes from file /work/riot/RIOT/examples/hello-world/bin/stm32f746g-disco/hello-world.elf in 0.776735s (41.198 KiB/s)

verified 9304 bytes in 0.079801s (113.857 KiB/s)

Info : Unable to match requested speed 2000 kHz, using 1800 kHz
Info : Unable to match requested speed 2000 kHz, using 1800 kHz
shutdown command invoked
Done flashing

With this PR, the board can be flashed reliably.

Issues/PRs references

Noticed that while working on #17437 and #17448

[aabadie]

@benpicco this is similar as #17462 (if you want to ACK it as well :) )

[benpicco]

Might this be needed for all f7 boards?

[aabadie]

Might this be needed for all f7 boards?

Maybe but I can't test the others :)