uart_tx_wait_idle: fix issue with last character not transmitted

- ROM function uart_tx_wait_idle may have a bug which causes the function to return before the final character is fully transmitted.
  This replaces uart_tx_wait_idle declaration with a static inline definition which fixes the issue.
- Also replaces the use of uart_tx_flush with uart_tx_wait_idle in esp_restart, to remove garbage in console output on restart.
- rtc_printf is temporary replaced with a no-op, pending a new release of librtc.a. Current release assumes that UART0 is used for output,
  and switches UART0 baud rate while doing frequency changes and printing some log output. This doesn’t work if a different UART is used for output.

components/esp32/include/rom/uart.h

@@ -18,6 +18,8 @@
 #include "esp_types.h"
 #include "esp_attr.h"
 #include "ets_sys.h"
+#include "soc/soc.h"
+#include "soc/uart_reg.h"
 
 #ifdef __cplusplus
 extern "C" {
@@ -260,11 +262,16 @@ void uart_tx_flush(uint8_t uart_no);
 /**
   * @brief Wait until uart tx full empty and the last char send ok.
   *
-  * @param  uint8_t uart_no : 0 for UART0, 1 for UART1.
+  * @param  uart_no : 0 for UART0, 1 for UART1, 2 for UART2
   *
-  * @return None.
+  * The function defined in ROM code has a bug, so we define the correct version
+  * here for compatibility.
   */
-void uart_tx_wait_idle(uint8_t uart_no);
+static inline void uart_tx_wait_idle(uint8_t uart_no) {
+    while(REG_GET_FIELD(UART_STATUS_REG(uart_no), UART_ST_UTX_OUT)) {
+        ;
+    }
+}
 
 /**
   * @brief Get an input char from message channel.

components/esp32/lib_printf.c

@@ -62,11 +62,8 @@ int phy_printf(const char* format, ...)
 
 int rtc_printf(const char* format, ...)
 {
-    va_list arg;
-    va_start(arg, format);
-    int res = lib_printf("rtc", format, arg);
-    va_end(arg);
-    return res;
+    // librtc.a printf temporary disabled due to UART baud rate switching bug.
+    return 0;
 }
 
 int wpa_printf(const char* format, ...)

components/esp32/system_api.c

@@ -105,10 +105,10 @@ void IRAM_ATTR esp_restart(void)
     Cache_Read_Disable(0);
     Cache_Read_Disable(1);
 
-    // Flush any data left in UART FIFO
-    uart_tx_flush(0);
-    uart_tx_flush(1);
-    uart_tx_flush(2);
+    // Flush any data left in UART FIFOs
+    uart_tx_wait_idle(0);
+    uart_tx_wait_idle(1);
+    uart_tx_wait_idle(2);
 
     // Reset wifi/bluetooth (bb/mac)
     SET_PERI_REG_MASK(DPORT_WIFI_RST_EN_REG, 0x1f);