Common: Add Random utilities

This makes it easier to generate random numbers or fill a buffer with
random data in a cryptographically secure way.

This also replaces existing usages of RNG functions in the codebase:

* <random> is pretty hard to use correctly, and std::random_device does
  not give enough guarantees about its results (it's
  implementation-defined, non cryptographically secure and could be
  deterministic on some platforms).
  Doing things correctly is error prone and verbose.

* rand() is terrible and should not be used especially in crypto code.

Source/Core/Common/Random.cpp

@@ -0,0 +1,49 @@
+// Copyright 2018 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#include "Common/Random.h"
+
+#include <mbedtls/entropy.h>
+#include <mbedtls/hmac_drbg.h>
+
+#include "Common/Assert.h"
+
+namespace Common::Random
+{
+class CSPRNG final
+{
+public:
+  CSPRNG()
+  {
+    mbedtls_entropy_init(&m_entropy);
+    mbedtls_hmac_drbg_init(&m_context);
+    const int ret = mbedtls_hmac_drbg_seed(&m_context, mbedtls_md_info_from_type(MBEDTLS_MD_SHA256),
+                                           mbedtls_entropy_func, &m_entropy, nullptr, 0);
+    ASSERT(ret == 0);
+  }
+
+  ~CSPRNG()
+  {
+    mbedtls_hmac_drbg_free(&m_context);
+    mbedtls_entropy_free(&m_entropy);
+  }
+
+  void Generate(void* buffer, std::size_t size)
+  {
+    const int ret = mbedtls_hmac_drbg_random(&m_context, static_cast<u8*>(buffer), size);
+    ASSERT(ret == 0);
+  }
+
+private:
+  mbedtls_entropy_context m_entropy;
+  mbedtls_hmac_drbg_context m_context;
+};
+
+static thread_local CSPRNG s_csprng;
+
+void Generate(void* buffer, std::size_t size)
+{
+  s_csprng.Generate(buffer, size);
+}
+}  // namespace Common::Random