Build system: Raise warning level

Default esp-idf builds now show -Wextra warnings (except for a few:
signed/unsigned comparison, unused parameters, old-style C declarations.)

CI building of examples runs with that level raised to -Werror, to catch
those changes going into the main repo.

components/bootloader_support/src/esp_image_format.c

@@ -17,7 +17,7 @@
 #include <esp_log.h>
 #include <bootloader_flash.h>
 
-const static char *TAG = "esp_image";
+static const char *TAG = "esp_image";
 
 #define SIXTEEN_MB 0x1000000
 #define ESP_ROM_CHECKSUM_INITIAL 0xEF

components/bootloader_support/src/secure_boot_signatures.c

@@ -45,10 +45,13 @@ esp_err_t esp_secure_boot_verify_signature(uint32_t src_addr, uint32_t length)
     sha_context sha;
     uint8_t digest[32];
     ptrdiff_t keylen;
-    const uint8_t *data, *digest_data;
-    uint32_t digest_len;
+    const uint8_t *data;
     const signature_block_t *sigblock;
     bool is_valid;
+#ifdef BOOTLOADER_BUILD
+    const uint8_t *digest_data;
+    uint32_t digest_len;
+#endif
 
     ESP_LOGD(TAG, "verifying signature src_addr 0x%x length 0x%x", src_addr, length);