Refactor slave_node application to support building for both bluepill_f103rb and
weact_stm32g431_core boards.
- Moved RTT-specific console and shell backend configurations from prj.conf
into board-specific .conf files (bluepill_f103rb.conf).
- Configured USART2 as console/shell for weact_stm32g431_core.
- Added Device Tree Overlay for weact_stm32g431_core to enable USART1 for Modbus
communication (PA9/PA10).
