lock path in barebox-state

[barebox+mailing at cookiesoft.de]

Hey everyone, 

we use barebox-state in conjunction with systemd on our platform. 
We removed all legacy programs and paths (in systemd terms), so is `/var/lock` removed as well. 

Lennart Poettering decribed it in a github issue why /var/lock is bad (and the reason why it's not pupolated per default): https://github.com/systemd/systemd/issues/15668#issuecomment-623069821 

So for now our option is to create /var/lock via tmpfiles so barebox-state can use them. 

I would like to see a upstream fix for this, where barebox uses its own directory, e.g. `/run/barebox/lock` or similar. 

I'm not sure if this breaks anything on older systems, maybe you can think about it for a second. 

Please let me know what you think the best approach would be. 

Greetings, 
Marcel