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

That's definitely helpful for debugging.

esp8266/Makefile

@@ -23,7 +23,9 @@ INC += -I../lib/timeutils
 INC += -I$(BUILD)
 INC += -I$(ESP_SDK)/include
 
-UART_OS = 1
+# UART for "os" messages. 0 is normal UART as used by MicroPython REPL,
+# 1 is debug UART (tx only).
+UART_OS = 0
 
 CFLAGS_XTENSA = -fsingle-precision-constant -Wdouble-promotion \
 	-D__ets__ -DICACHE_FLASH \

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);
 }