esp32: Skip validation of image on boot from deepsleep.

sdkconfig.base: Add CONFIG_BOOTLOADER_SKIP_VALIDATE_IN_DEEP_SLEEP=y.
This reduces time to boot from deepsleep by at least 200ms and can
provide significant power savings for deepsleep-based battery
applications.

docs/library/esp32.rst: Add note cautioning not to enter deepsleep after
changing the boot partition, without first performing a hard reset.

Signed-off-by: Glenn Moloney <glenn.moloney@gmail.com>

docs/library/esp32.rst

@@ -114,6 +114,11 @@ methods to enable over-the-air (OTA) updates.
 
     Sets the partition as the boot partition.
 
+    .. note:: Do not enter :func:`deepsleep<machine.deepsleep>` after changing
+       the OTA boot partition, without first performing a hard
+       :func:`reset<machine.reset>` or power cycle. This ensures the bootloader
+       will validate the new image before booting.
+
 .. method:: Partition.get_next_update()
 
     Gets the next update partition after this one, and returns a new Partition object.

ports/esp32/boards/sdkconfig.base

@@ -13,6 +13,7 @@ CONFIG_APP_EXCLUDE_PROJECT_NAME_VAR=y
 
 # Bootloader config
 CONFIG_BOOTLOADER_LOG_LEVEL_WARN=y
+CONFIG_BOOTLOADER_SKIP_VALIDATE_IN_DEEP_SLEEP=y
 
 # Change default log level to "ERROR" (instead of "INFO")
 CONFIG_LOG_DEFAULT_LEVEL_INFO=n