Kernel/Commandline: Allow the user to specify an embedded string

This is by default left empty, so people won't run the kernel in a mode
which they didn't want to. The embedded string will override the
supplied commandline from the bootloader, which is good for debugging
sessions.

This change seemed important for me, because I debug the kernel on bare
metal with iPXE, and every change to the commandline meant that I needed
rewrite a new iPXE USB image with a modified iPXE script.

Kernel/CommandLine.h

@@ -77,6 +77,9 @@ public:
 private:
     CommandLine(const String&);
 
+    void add_arguments(const Vector<String>& args);
+    void build_commandline(const String& cmdline_from_bootloader);
+
     String m_string;
     HashMap<String, String> m_params;
 };