[common] Make Serial RX buffer size configurable, increase default to 256 bytes

[hn]

Increase RX buffer from 64 bytes to 256 bytes (ESPHome UART bus component default). Other sizes can easily be defined in ESPHome YAML e.g. like this:

bk72xx:
  board: cbu
  framework:
    options:
      LT_SERIAL_BUFFER_SIZE: 512

Preparatory work for esphome/issues#6240. ESPHome could change LT_SERIAL_BUFFER_SIZE whenever rx_buffer_size: is changed.

Also covertly increases the send buffer for SoftwareSerial, but that should have no side effects.

[hn]

Basic tests done with cbu and rtl8710bx modules.

[hn]

Withdraw Relativize the above statement: rx_buffer_size: is a per-device (per-UART) setting, while LT_SERIAL_BUFFER_SIZE would be a global setting. That would be somewhat inconsistent.

Anyway, it is an advantage if the standard buffer size is increased and made adjustable.

[kuba2k2]

I know it's a per-UART setting, but it's the most logical place to put it. So unless rx_buffer_size: controls ESPHome's internal buffer, it should control LibreTiny's global buffer. If there is more than one UART component used (likely never), prohibit changing the buffer size on one of them only.