esp8266: Let esp8266 "os" messages go to standard (REPL) UART.

That's definitely helpful for debugging.

esp8266/uart.c

@@ -205,7 +205,7 @@ void ICACHE_FLASH_ATTR uart_init(UartBautRate uart0_br, UartBautRate uart1_br) {
     uart_config(UART1);
     ETS_UART_INTR_ENABLE();
 
-    // install uart1 putc callback
+    // install handler for "os" messages
     os_install_putc1((void *)uart_os_write_char);
 }