getting-started: Reduce RSA sign stack usage

On targets with limited stack available to the main thread, our example
can run out of stack space when doing RSA encryption. This can lead to
stack overflows which cause undefined behavior. Move the RSA key and
signature to static RAM instead of stack to reduce the stack footprint
of message signing done by the example.

Author: Patater

getting-started/main.cpp

@@ -142,9 +142,10 @@ static void sign_a_message_using_rsa(void)
 {
     psa_status_t status;
     psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
-    uint8_t key[] = RSA_KEY;
+    static const uint8_t key[] = RSA_KEY;
     uint8_t hash[] = "INPUT_FOR_SIGN";
-    uint8_t signature[PSA_ASYMMETRIC_SIGNATURE_MAX_SIZE] = {0};
+    /* 'signature' is static to reduce stack usage. */
+    static uint8_t signature[PSA_ASYMMETRIC_SIGNATURE_MAX_SIZE] = {0};
     size_t signature_length;
     psa_key_handle_t handle;