Kernel/Loader: Grab the system mode from the NCCH ExHeader.

3dsx and elf files default to system mode 2 (96MB allocated to the application).

This allows Home Menu to boot without modifications.
Closes #1849

src/core/hle/kernel/kernel.h

@@ -286,8 +286,8 @@ private:
 
 extern HandleTable g_handle_table;
 
-/// Initialize the kernel
-void Init();
+/// Initialize the kernel with the specified system mode.
+void Init(u32 system_mode);
 
 /// Shutdown the kernel
 void Shutdown();