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The renesas-ra port supports calling a handler to be called on every byte
received by UART.  For consistency with other ports, the symbol IRQ_RX
is added as the trigger name.

Side change: Add the received UART data to the REPL input buffer only if it
is the REPL UART.  Otherwise, every UART would act as REPL input.
Signed-off-by: robert-hh <robert@hammelrath.com>