change(bootloader): honor SECURE_BOOT_ALLOW_UNUSED_DIGEST_SLOTS during first boot

Previously the bootloader unconditionally revoked unused secure boot key
digest slots while permanently enabling secure boot on the first boot,
ignoring CONFIG_SECURE_BOOT_ALLOW_UNUSED_DIGEST_SLOTS. Now the config is
honored on this path too: when set, the unused digest slots are left
un-revoked. This is safe as long as the debug and download interfaces are
disabled.

Update the Kconfig help and the Secure Boot v2 guide (en and zh_CN)
accordingly.

components/bootloader/Kconfig.projbuild

@@ -1012,13 +1012,8 @@ menu "Security features"
                 Revoking unused digest slots makes ensures that no trusted keys can be added later by an attacker.
                 If set, it means that you have a plan to use unused digests slots later.
 
-                Note that if you plan to enable secure boot during the first boot up, the bootloader will intentionally
-                revoke the unused digest slots while enabling secure boot, even if the above config is enabled because
-                keeping the unused key slots un-revoked would a security hazard.
-                In case for any development workflow if you need to avoid this revocation, you should enable
-                secure boot externally (host based mechanism) rather than enabling it during the boot up,
-                so that the bootloader would not need to enable secure boot and thus you could avoid its revocation
-                strategy.
+                This config is honored both at runtime in the app and while enabling secure boot during the first
+                boot up in the bootloader. When set, the unused digest slots are left un-revoked in both cases.
 
         config SECURE_BOOT_SKIP_WRITE_PROTECTION_SCA
             bool "Skip write-protection of SECURE_FLASH_PSEUDO_ROUND_FUNC_STRENGTH"

components/bootloader_support/src/secure_boot_v2/secure_boot.c

@@ -347,8 +347,12 @@ static esp_err_t check_and_generate_secure_boot_keys(const esp_image_metadata_t
     if (boot_key_digests.num_digests < SECURE_BOOT_NUM_BLOCKS) {
         /* The revocation index can be 0, 1, 2. Bootloader count can be 1,2,3. */
         for (unsigned i = boot_key_digests.num_digests; i < SECURE_BOOT_NUM_BLOCKS; i++) {
+#ifndef CONFIG_SECURE_BOOT_ALLOW_UNUSED_DIGEST_SLOTS
             ESP_LOGI(TAG, "Revoking empty key digest slot (%d)...", i);
             esp_efuse_set_digest_revoke(i);
+#else
+            ESP_LOGW(TAG, "Unused key digest slot (%d) left un-revoked due to the config SECURE_BOOT_ALLOW_UNUSED_DIGEST_SLOTS", i);
+#endif
         }
     }
 #endif // SOC_EFUSE_REVOKE_BOOT_KEY_DIGESTS

docs/en/security/secure-boot-v2.rst

@@ -741,9 +741,7 @@ Secure Boot Best Practices
 
     .. note::
 
-        Note that enabling the config :ref:`CONFIG_SECURE_BOOT_ALLOW_UNUSED_DIGEST_SLOTS` only makes sure that the **app** does not revoke the unused digest slots.
-        But if you plan to enable secure boot during the fist boot up, the bootloader will intentionally revoke the unused digest slots while enabling secure boot, even if the above config is enabled. Because keeping the unused key slots unrevoked would be a security hazard.
-        In case for any development workflow if you need to avoid this revocation, you should :ref:`enable-secure-boot-v2-externally`, rather than enabling it during the boot up, so that the bootloader would not need to enable secure boot, and thus you could avoid its revocation strategy.
+        Enabling the config :ref:`CONFIG_SECURE_BOOT_ALLOW_UNUSED_DIGEST_SLOTS` keeps the unused digest slots un-revoked in both cases: at runtime in the **app**, and in the **bootloader** when secure boot is enabled during the first boot up. Note that leaving unused key slots un-revoked could pose a security risk, unless the debug and download interfaces are completely disabled and remote interfaces are fully audited for security risks.
 
     Conservative Approach
     ~~~~~~~~~~~~~~~~~~~~~~

docs/zh_CN/security/secure-boot-v2.rst

@@ -741,9 +741,7 @@ Secure Boot v2 签名验证也可以在 OTA 更新期间验证数据分区镜像
 
     .. note::
 
-        请注意，启用配置 :ref:`CONFIG_SECURE_BOOT_ALLOW_UNUSED_DIGEST_SLOTS` 只能确保 **应用程序** 不会撤销未使用的摘要槽。
-        若想在设备首次启动时启用安全启动，那么即使启用了上述配置，引导加载程序也会在启用安全启动时撤销未使用的摘要槽，因为保留未使用的密钥槽会构成安全隐患。
-        如果在开发流程中需要保留未使用摘要槽，则应从外部启用安全启动 (:ref:`enable-secure-boot-v2-externally`)，而不是在启动设备时启用安全启动，这样引导加载程序就无需启用安全启动，从而避免安全隐患。
+        启用配置 :ref:`CONFIG_SECURE_BOOT_ALLOW_UNUSED_DIGEST_SLOTS` 后，未使用的摘要槽在两种情况下都将保持未撤销状态：在 **应用程序** 运行时，以及在设备首次启动时由 **引导加载程序** 启用安全启动时。请注意，除非调试接口和下载接口已完全禁用，且远程接口已针对安全风险进行全面审计，否则保留未使用的密钥槽可能构成安全风险。
 
     保守方法
     ~~~~~~~~