sdmmc: fix explanation of flash voltage regulator configuration efuses

- low level on GPIO12 corresponds to 3.3V output, not 1.8V
- add espefuse.py commands to burn efuses
- add XPD_SDIO_REG to the list of efuses which must be set

Reported on the forum: http://esp32.com/viewtopic.php?f=2&t=849&start=10#p4170

examples/27_sd_card/README.md

@@ -40,9 +40,20 @@ GPIO12 is used as a bootstrapping pin to select output voltage of an internal re
 
 - For boards which don't use the internal regulator, GPIO12 can be pulled high.
 - On boards which use the internal regulator and a 3.3V flash chip, GPIO12 should be pulled up high, which is compatible with SD card operation.
-- For boards which use 1.8V flash chip, GPIO12 needs to be low at reset.
+- For boards which use 3.3V flash chip, GPIO12 needs to be low at reset.
     * In this case, internal pullup can be enabled using a `gpio_pullup_en(GPIO_NUM_12);` call. Most SD cards work fine when an internal pullup on GPIO12 line is enabled. Note that if ESP32 experiences a power-on reset while the SD card is sending data, high level on GPIO12 can be latched into the bootstrapping register, and ESP32 will enter a boot loop until external reset with correct GPIO12 level is applied.
-    * Another option is to program flash voltage selection efuses: set `SDIO_TIEH=0` and `SDIO_FORCE=1`. This will permanently select 1.8v output voltage for the internal regulator, and GPIO12 will not be used as a bootstrapping pin anymore. Then it is safe to connect a pullup resistor to GPIO12. This option is suggested for production use.
+    * Another option is to program flash voltage selection efuses: set `XPD_SDIO_TIEH=1`, `XPD_SDIO_FORCE=1`, and `XPD_SDIO_REG = 1`. This will permanently select 3.3V output voltage for the internal regulator, and GPIO12 will not be used as a bootstrapping pin anymore. Then it is safe to connect a pullup resistor to GPIO12. This option is suggested for production use.
+
+The following commands can be used to program flash voltage selection efuses **to 3.3V**:
+
+```sh
+	# Override flash regulator configuration using efuses
+	components/esptool_py/esptool/espefuse.py burn_efuse XPD_SDIO_FORCE
+	# Select 3.3V output voltage
+	components/esptool_py/esptool/espefuse.py burn_efuse XPD_SDIO_TIEH
+	# Enable internal voltage regulator
+	components/esptool_py/esptool/espefuse.py burn_efuse XPD_SDIO_REG
+```
 
 ## 4-line and 1-line modes