ESP32: Add support for the 3/4 key encoding scheme

* Add calculated redundancy values when writing eFuse blocks 1-3
 * Perform 192 -> 256 bit key extension when encrypting images for flashing

Fixes https://github.com/cesanta/mos-tool/issues/37

CL: mos: ESP32: Add support for the 3/4 key encoding scheme

PUBLISHED_FROM=16639fb25d5200550ad2097929239d896e188b5b

src/common/platforms/esp/stub_flasher.c

@@ -470,6 +470,27 @@ uint8_t cmd_loop(void) {
         resp = 0;
         break;
       }
+      case CMD_READ_REG: {
+        len = SLIP_recv(args, sizeof(args));
+        if (len == 4) {
+          uint32_t value = READ_PERI_REG((uint32_t *) args[0]);
+          SLIP_send(&value, sizeof(value));
+          resp = 0;
+        } else {
+          resp = 0x91;
+        }
+        break;
+      }
+      case CMD_WRITE_REG: {
+        len = SLIP_recv(args, sizeof(args));
+        if (len == 8) {
+          WRITE_PERI_REG((uint32_t *) args[0], args[1]);
+          resp = 0;
+        } else {
+          resp = 0xa1;
+        }
+        break;
+      }
     }
     SLIP_send(&resp, 1);
   } while (cmd != CMD_BOOT_FW && cmd != CMD_REBOOT);

src/common/platforms/esp/stub_flasher.h

@@ -91,6 +91,24 @@ enum stub_cmd {
    * Output: arguments.
    */
   CMD_ECHO = 8,
+
+  /*
+   * Read register value.
+   *
+   * Args: register address.
+   * Input: None.
+   * Output: register value.
+   */
+  CMD_READ_REG = 9,
+
+  /*
+   * Write register value.
+   *
+   * Args: register address, value.
+   * Input: None.
+   * Output: None.
+   */
+  CMD_WRITE_REG = 10,
 };
 
 #endif /* CS_COMMON_PLATFORMS_ESP8266_STUBS_STUB_FLASHER_H_ */