Use accelerated hashing for verification

Modern CPUs have special instructions to accelerate computing
SHA hashes.
The Qt QCryptographicHash code is standard C, so not taking
advantage of those though.
Outsource the hashing to OpenSSL that does.

Shaves off some seconds during verification stage.

acceleratedcryptographichash.cpp

@@ -0,0 +1,37 @@
+/*
+ * Use OpenSSL for hashing as their code is more optimized than Qt's
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ * Copyright (C) 2020 Raspberry Pi (Trading) Limited
+ */
+
+#include "acceleratedcryptographichash.h"
+
+AcceleratedCryptographicHash::AcceleratedCryptographicHash(QCryptographicHash::Algorithm method)
+{
+    if (method != QCryptographicHash::Sha256)
+        throw std::runtime_error("Only sha256 implemented");
+
+    SHA256_Init(&_sha256);
+}
+
+AcceleratedCryptographicHash::~AcceleratedCryptographicHash()
+{
+}
+
+void AcceleratedCryptographicHash::addData(const char *data, int length)
+{
+    SHA256_Update(&_sha256, data, length);
+}
+
+void AcceleratedCryptographicHash::addData(const QByteArray &data)
+{
+    addData(data.constData(), data.size());
+}
+
+QByteArray AcceleratedCryptographicHash::result()
+{
+    unsigned char binhash[SHA256_DIGEST_LENGTH];
+    SHA256_Final(binhash, &_sha256);
+    return QByteArray((char *) binhash, sizeof binhash);
+}