fix(esp32h2): program use_hardware_k efuse bit for ECDSA key purpose

In ESP32-H2, the ECDSA peripheral by default uses the TRNG (hardware)
generated k value but it can be overridden to software supplied k.
This can happen through by overriding the `ECDSA_SOFTWARE_SET_K` bit
in the configuration register. Even though the HAL API is not exposed
for this but still it could be achieved by direct register
programming. And for this scenario, if sufficiently random k is not
supplied by the software then it could posses a security risk.

In this change, we are unconditionally programming the efuse
`ESP_EFUSE_ECDSA_FORCE_USE_HARDWARE_K` bit during startup security
checks itself. Additionally, same is ensured in the `esp_efuse_write_key`
API as well. This always enforces the hardware k mode in the ECDSA
peripheral and ensures strongest possible security.

components/esp_system/startup.c

@@ -27,6 +27,7 @@
 #include "esp_newlib.h"
 #include "esp_timer.h"
 #include "esp_efuse.h"
+#include "esp_efuse_table.h"
 #include "esp_flash_encrypt.h"
 #include "esp_secure_boot.h"
 #include "esp_xt_wdt.h"
@@ -348,6 +349,15 @@ static void do_core_init(void)
     esp_secure_boot_init_checks();
 #endif
 
+#if SOC_EFUSE_ECDSA_USE_HARDWARE_K
+    if (esp_efuse_find_purpose(ESP_EFUSE_KEY_PURPOSE_ECDSA_KEY, NULL)) {
+        // ECDSA key purpose block is present and hence permanently enable
+        // the hardware TRNG supplied k mode (most secure mode)
+        err = esp_efuse_write_field_bit(ESP_EFUSE_ECDSA_FORCE_USE_HARDWARE_K);
+        assert(err == ESP_OK && "Failed to enable ECDSA hardware k mode");
+    }
+#endif
+
 #if CONFIG_SECURE_DISABLE_ROM_DL_MODE
     err = esp_efuse_disable_rom_download_mode();
     assert(err == ESP_OK && "Failed to disable ROM download mode");