RP2040: enable entropy source

Fixes #437

Author: facchinm

variants/NANO_RP2040_CONNECT/conf/mbed_app.json

@@ -2,6 +2,7 @@
   "target_overrides": {
     "*": {
       "target.printf_lib": "std",
+      "target.macros_add": [ "MBEDTLS_ENTROPY_NV_SEED" ],
       "platform.stdio-buffered-serial": false,
       "platform.stdio-baud-rate": 115200,
       "platform.default-serial-baud-rate": 115200,

variants/NANO_RP2040_CONNECT/variant.cpp

@@ -94,4 +94,16 @@ void _ontouch1200bps_() {
   reset_usb_boot(1 << digitalPinToPinName(LED_BUILTIN), 0);
 }
 
+// Provide stubs for mbedtls entropy source
+// The functions may be implemented using ECCx06 crypto chip
+extern "C" {
+  WEAK int mbedtls_platform_std_nv_seed_read( unsigned char *buf, size_t buf_len ) {
+    return -1;
+  }
+
+  WEAK int mbedtls_platform_std_nv_seed_write( unsigned char *buf, size_t buf_len ) {
+    return -1;
+  }
+}
+
 #endif