bootloader_support: initialize mbedtls_ctr_drbg_context per mbedtls v2.16.5 requirement

In commit 02d2903e39e87f185c5af9029b2c09c0943b8b05, mbedtls was
updated to release v2.16.5, where it was made mandatory to initialize
mbedtls_ctr_drbg_context before using same. It was fixed in wpa supplicant
but missed out in secure boot v2 verification code. This commit
fixes that.

components/bootloader_support/src/idf/secure_boot_signatures.c

@@ -207,6 +207,7 @@ esp_err_t esp_secure_boot_verify_rsa_signature_block(const ets_secure_boot_signa
     }
 
     mbedtls_entropy_init(&entropy);
+    mbedtls_ctr_drbg_init(&ctr_drbg);
     ret = mbedtls_ctr_drbg_seed(&ctr_drbg, mbedtls_entropy_func, &entropy, NULL, 0);
     if (ret != 0) {
         ESP_LOGE(TAG, "mbedtls_ctr_drbg_seed returned -0x%04x\n", ret);