[core] Allow changing SDK, logger & Serial output ports

docs/config.md

@@ -62,9 +62,19 @@ Families should generally call i.e. WiFiClient debugging for client-related code
 
 Options for controlling default UART log output.
 
+- `LT_UART_DEFAULT_PORT` (unset) - default output port for all messages (SDK, LT logger, Serial class); can be 0, 1 or 2
+- `LT_UART_DEFAULT_LOGGER` (unset) - override default output port for LT logger only
+- `LT_UART_DEFAULT_SERIAL` (unset) - override default output port for `Serial` class (without a number)
 - `LT_UART_SILENT_ENABLED` (1) - enable auto-silencing of SDK "loggers"; this makes the serial output much more readable, but can hide some error messages
 - `LT_UART_SILENT_ALL` (0) - disable all SDK output (LT output and logger still work)
 
+!!! info
+    Values 0, 1 and 2 correspond to physical UART port numbers (refer to board pinout for the available ports).
+
+    Serial class instances (`Serial0`, `Serial1`, `Serial2`) use the respective port numbers for printing.
+
+    If `LT_UART_DEFAULT_LOGGER` is not set, it is chosen by the family code - whichever port is most appropriate (i.e. LOG_UART (2) on Realtek, RX2/TX2 on Beken).
+
 ### Family feature config
 
 These options are selectively set by all families, as part of the build process. They are used for enabling LT core API parts, if the family has support for it.