Add make rule for flashing with ST-LINK: stflash

[geolessel]

I noticed that there was a make rule of %.stlink-flash but nothing called that particular code path. This adds a rule of stflash that flashes the binary with ST-LINK.

Please note that I am not really familiar with Makefiles. This is what I had to do to get this working after looking through the file. Please let me know if there is a better name than stflash or if there are other ways to make this better.