feat: add stlink_sn option for swd upload

It allows to specify serial number of the ST-Link device
(if multiple ST-Link devices are connected)

Goal is to use it over the boards.local.txt.
Ex:
Nucleo_64.menu.pnum.NUCLEO_C562RE.upload.stlink_sn=003100344142501020353451

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm