Consolidate CDC, UART and NUS stdio interfaces into the one handler.
Allows any/all of them to be enabled separately.
Updates UART REPL to use similar define to other platforms:
`MICROPY_HW_ENABLE_UART_REPL`.
USB now uses the shared/tinyusb CDC implementation.
Signed-off-by: Andrew Leech <andrew@alelec.net>