Add st-flash flash target (#9964)

* Add `st-flash` flash target

Add support for flashing the firmware via the `st-flash` utility from
the STLink Tools package (https://github.com/stlink-org/stlink).

* Add `st-flash` to the `qmk flash -b` output

lib/python/qmk/cli/flash.py

@@ -27,6 +27,7 @@ def print_bootloader_help():
     cli.echo('\tdfu-util-split-left')
     cli.echo('\tdfu-util-split-right')
     cli.echo('\tst-link-cli')
+    cli.echo('\tst-flash')
     cli.echo('For more info, visit https://docs.qmk.fm/#/flashing')