@scalz well, that's nice that you know what you want and can get it, but that doesn't mean that's suitable for everybody else. That's good to have a choice, but when you need it done, you better find a way to get it done easier and faster.
So I went to local store, bought second st-link clone for 4$ and then spent a few hours figuring out, how to properly flash it. The day after I could unlock my Ebyte e73 board, with newly acquired BMP.
If I'd took the route suggested by you, I would spend minimum 30$ and 1-2 weeks of waiting time. And then I would still need to figure out how to work with that proprietary software.
I am far from experienced programmer, so I can't get use of BMP's full potential, but now I have a very useful tool in my arsenal, that can be used with variety of different chips.
Also, in my opinion, the main advantage of opensource not the software itself, but a community. Your experience may vary, but I'd say you have better chances finding solution in opensource community, than on commercial software forum, unless you have some kind of paid support subscription.

@sergio-rius well, I guess you are right. I revisited description of BMP and it says, that it runs GBD server on itself and eliminates need for STlink server. So I guess you could do all the same with just an st-link...however it seems that it would require more setup and I don't think that it will be easier than flashing BMP firmware
Just a snippet from their wiki:
"There are a few advantages of the Black Magic Probe. BMP is open-source, meaning that you can look inside it if you need or want to. We are getting support for new ARM Cortex-M based chips on a regular basis, so you are not limited to just the STM32. We have preliminary support for Cortex-A this will result in the ability to use the probe with Raspberry PI and Beagle Bone Black and many others. The Black Magic Probe also supports JTAG not only SWD, because not all microcontrollers use SWD. Also JTAG makes it possible to chain together more than one microcontroller. The GDB server is implemented on the probe itself, this means we do not use some proprietary protocol to talk to your debugger software, making the setup more repeatable and removing the need for custom config files."

@sergio-rius but debugger is running on a computer side anyway, as far as I understand. And BMP is the "thing" that connects it to MCU via SWD.

@neverdie and how do you suggest connecting SWD and SWCLK lines from MCU to computer, not using the "probe"?

@sergio-rius I too have struggled to flash st-link clone with bmp firmware, but eventually I managed to do it. Unfortunately I don't remember what exactly I did, but I remember that I came to a solution when decided to try another st-link as a destination. If I remember correctly, there are different stm32 chips on these boards and some of them have insufficient amount of flash memory for bmp firmware.

arm-none-eabi-gdb package is not available in ubuntu's repositories starting from 18.04. You can either download toolchain from eclipse's github archive or try to install older package as described here.

Also, I used info from this blogs when flashed my st-link:
http://blog.linuxbits.io/2016/02/15/cheap-chinese-st-link-v-2-programmer-converted-to-black-magic-probe-debugger/
http://nuft.github.io/arm/2015/08/24/blackmagic-stlink.html

As for buying stuff, don't forget you can buy ready to use BMP from their creators too

Though the power bug @Nca78 mentioned is easily bypassed with proper software settings. Just don't rely on built in sleep function, because in case of nfr51822 it doesn't disable all hardware and drains power.

@scalz With official support from arduino and massive amount of chinese clone boards and much more powerful hardware I think in a year new nano board can become a new standard. Not to mention that single board which already has everything to work with mysensors is something many people where looking for for years.
I understand the time constrains, and am always thankful for a dev's work for community's good

@nagelc I think Sandeep Mistry is working on new arduino core, based on mbed. As you can see he is titled as "Senior Software Engineer at Arduino" here. So I guess Mysensors community would need to move to new core eventually, or to fork Sandeep's core and develop it alone. I would prefer the first choice.

Well, I never returned to that topic, because I didn't need it. I think it still isn't implemented, but you have my code from that post, so you can try to modify your copy of Mysensors and it should work. If I remember correctly that was pretty simple modification, new version has probably changed, but not so much I guess.

@mfalkvidd I understand. Maybe we could ask author of the plugin what he sinks about it, if you know his name on this forum and can tag him? There is possibility to manually config some parts in HA, but mysensors plugin uses config files only to set general options for gateway, there is no options for individual nodes in config (except changing node name). Maybe they could be added, but I imagine that this will brake the approach the author of the plugin has to it.