main/busybox: Add serial consoles to securetty

Add to securetty file the serial consoles related to the following devices:

- NVIDIA Jetson Xavier NX developer kit
- Lenovo ThinkEdge SE70
- phyBOARD-Pollux board (i.MX8M Plus SoC)
- Advantech EPC-R3720 (i.MX8M Plus SoC)
- Rockchip RK3399 developer kit

Signed-off-by: Renê de Souza Pinto <rene@renesp.com.br>