Restructure asset processor

tools/asset_processor/util.cpp

@@ -0,0 +1,29 @@
+#include "util.h"
+#include <iostream>
+
+void check_call(const std::vector<std::string>& cmd) {
+    std::string cmdstr;
+    bool first = true;
+    for (const auto& segment : cmd) {
+        if (first) {
+            first = false;
+        } else {
+            cmdstr += " ";
+        }
+        cmdstr += segment;
+    }
+    int code = system(cmdstr.c_str());
+    if (code != 0) {
+        std::cerr << cmdstr << " failed with return code " << code << std::endl;
+        std::exit(1);
+    }
+}
+
+// https://github.com/nlohmann/json/issues/642#issuecomment-311937344
+std::string to_string(const nlohmann::json& j) {
+    if (j.type() == nlohmann::json::value_t::string) {
+        return j.get<std::string>();
+    }
+
+    return j.dump();
+}