echoserver: add Ed25519 sample public keys

The example server loads hansel's and gretel's Ed25519 public keys when
neither RSA nor ECDSA is compiled in, so public key auth works there
instead of falling back to passwords. That arm is the "#if" and the
userEcc choice the "#else", so no empty if/else is left behind.

examples/echoserver/echoserver.c

@@ -2712,6 +2712,17 @@ static const char samplePublicKeyEccBuffer[] =
 #endif /* WOLFSSH_TPM */
 #endif /* WOLFSSH_NO_RSA */
 
+/* Ed25519 is the only signing algorithm left when neither RSA nor ECDSA
+ * is compiled in, so the server needs sample keys of its own. */
+#if defined(WOLFSSH_NO_RSA) && defined(WOLFSSH_NO_ECDSA) && \
+    !defined(WOLFSSH_NO_ED25519)
+static const char samplePublicKeyEd25519Buffer[] =
+    "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIHTSoBZIJBO2V0Jb2OWyMWNbkD"
+    "d6ReDfKxnrAPlbPuCe hansel\n"
+    "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIFD8Bwir++gzNJmif9ooAZdaRi"
+    "sFZjlp9XU2seaec7/m gretel\n";
+#endif
+
 #ifdef WOLFSSH_ALLOW_USERAUTH_NONE
 
 static const char sampleNoneBuffer[] =
@@ -4313,6 +4324,13 @@ THREAD_RETURN WOLFSSH_THREAD echoserver_test(void* args)
         keyLoadBuf[bufSz] = 0;
         LoadPasswordBuffer(keyLoadBuf, bufSz, &pwMapList);
 
+        #if defined(WOLFSSH_NO_RSA) && defined(WOLFSSH_NO_ECDSA)
+        /* Ed25519 is the only sample key left, so -e has nothing to pick
+         * between and is ignored. */
+        #ifndef WOLFSSH_NO_ED25519
+            bufName = samplePublicKeyEd25519Buffer;
+        #endif
+        #else
         if (userEcc) {
         #ifndef WOLFSSH_NO_ECDSA
             bufName = samplePublicKeyEccBuffer;
@@ -4327,6 +4345,7 @@ THREAD_RETURN WOLFSSH_THREAD echoserver_test(void* args)
             #endif
         #endif
         }
+        #endif
         if (bufName != NULL) {
             bufSz = (word32)WSTRLEN(bufName);
             WMEMCPY(keyLoadBuf, bufName, bufSz);