[RZ_A1LU] Fix TRNG function

Related to the review of #5857, I fixed the TRNG function for GR-LYCHEE.
- I modified to zeroize "recv_data" before the function return.
- I added the processing that check the return value of I2C.read function. If return value is error, "output" is zeroized before function return.
- In trng_get_bytes_esp32 function, there is a time lag in the period from ESP32 reset to start working, error may occur when "Write" is called. Thus, I added a retry counter due to address this concern. There is not this counter for "Read" since it is called after "Write".

targets/TARGET_RENESAS/TARGET_RZ_A1XX/TARGET_GR_LYCHEE/trng_api_esp32.cpp

@@ -19,6 +19,12 @@
 #include "platform/mbed_wait_api.h"
 
 #define ESP32_I2C_ADDR    (0x28<<1)
+#define RETRY_CNT_MAX     (20)
+
+/* Implementation that should never be optimized out by the compiler */
+static void mbedtls_zeroize( void *v, size_t n ) {
+    volatile unsigned char *p = (unsigned char *)v; while( n-- ) *p++ = 0;
+}
 
 extern "C" void trng_init_esp32(void)
 {
@@ -59,27 +65,35 @@ extern "C" int trng_get_bytes_esp32(uint8_t *output, size_t length, size_t *outp
     char recv_data[4];
     size_t idx = 0;
     int i;
-    int err_cnt = 0;
+    int retry_cnt = 0;
 
-    while (idx < length) {
+    if ((output == NULL) || (output_length == NULL)) {
+        return -1;
+    }
+
+    while ((retry_cnt < RETRY_CNT_MAX) && (idx < length)) {
         send_data[0] = 0;
-        ret = mI2c.write(ESP32_I2C_ADDR, send_data, 1);
+        ret = mI2c.write(ESP32_I2C_ADDR, send_data, sizeof(send_data));
         if (ret == 0) {
-            mI2c.read(ESP32_I2C_ADDR, recv_data, sizeof(recv_data));
-            for (i = 0; (i < 4) && (idx < length); i++) {
-                output[idx++] = recv_data[i];
-            }
-        } else {
-            err_cnt++;
-            if (err_cnt >= 20) {
-                break;
+            ret = mI2c.read(ESP32_I2C_ADDR, recv_data, sizeof(recv_data));
+            if (ret == 0) {
+                for (i = 0; (i < sizeof(recv_data)) && (idx < length); i++) {
+                    output[idx++] = recv_data[i];
+                }
             }
+        }
+        if (ret != 0) {
+            retry_cnt++;
             wait_ms(100);
         }
     }
-    if (output_length != NULL) {
-        *output_length = idx;
+    if (retry_cnt >= RETRY_CNT_MAX) {
+        idx = 0;
+        mbedtls_zeroize(output, length);
     }
+    *output_length = idx;
+
+    mbedtls_zeroize(recv_data, sizeof(recv_data));
 
     return (idx != 0 ? 0 : -1);
 }