scripts: twister: fix regenerate serial device with unknown platform

Since BOOT-SERIAL isn't a recognized STMicroelectronics product name,
when Twister saves the new map.yaml, it will also check if the previous
map.yaml had the 'SERIAL-BOOT' product name and attach the corresponding
serial device.It won't generate a new one with an unknown platform name.


Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>