[mkbundle] Add support for generating monodroid compatible mkbundle code at runtime (#4728)

monodroid used to build its own version of mkbundle since it
requires slightly different code than the desktop. This
logic was protected in a #if block. Now that monodroid is
trying to make use of the shipped mkbundle rather than use
its own we need to turn this logic on at runtime.

A new parameter has been added

	--bundled-header

When this option is passed, the code which skips including

	mono/metadata/mono-config.h

will be skipped in favour of defining the required structures
and entry points directly.

mcs/tools/mkbundle/mkbundle.cs

@@ -49,6 +49,7 @@ class MakeBundle {
 	static string machine_config_file = null;
 	static string config_dir = null;
 	static string style = "linux";
+	static bool bundled_header = false;
 	static string os_message = "";
 	static bool compress;
 	static bool nomain;
@@ -347,6 +348,9 @@ class MakeBundle {
 				else
 					environment.Add (env.Substring (0, p), env.Substring (p+1));
 				break;
+			case "--bundled-header":
+				bundled_header = true;
+				break;
 			default:
 				sources.Add (args [i]);
 				break;
@@ -700,10 +704,10 @@ class MakeBundle {
 			using (StreamWriter tc = new StreamWriter (File.Create (temp_c))) {
 			string prog = null;
 
-#if XAMARIN_ANDROID
-			tc.WriteLine ("/* This source code was produced by mkbundle, do not edit */");
-			tc.WriteLine ("\n#ifndef NULL\n#define NULL (void *)0\n#endif");
-			tc.WriteLine (@"
+			if (bundled_header) {
+				tc.WriteLine ("/* This source code was produced by mkbundle, do not edit */");
+				tc.WriteLine ("\n#ifndef NULL\n#define NULL (void *)0\n#endif");
+				tc.WriteLine (@"
 typedef struct {
 	const char *name;
 	const unsigned char *data;
@@ -712,10 +716,10 @@ typedef struct {
 void          mono_register_bundled_assemblies (const MonoBundledAssembly **assemblies);
 void          mono_register_config_for_assembly (const char* assembly_name, const char* config_xml);
 ");
-#else
-			tc.WriteLine ("#include <mono/metadata/mono-config.h>");
-			tc.WriteLine ("#include <mono/metadata/assembly.h>\n");
-#endif
+			} else {
+				tc.WriteLine ("#include <mono/metadata/mono-config.h>");
+				tc.WriteLine ("#include <mono/metadata/assembly.h>\n");
+			}
 
 			if (compress) {
 				tc.WriteLine ("typedef struct _compressed_data {");
@@ -1238,6 +1242,7 @@ void          mono_register_config_for_assembly (const char* assembly_name, cons
 				   "    --skip-scan          Skip scanning assemblies that could not be loaded (but still embed them).\n" +
 				   "    --i18n ENCODING      none, all or comma separated list of CJK, MidWest, Other, Rare, West.\n" +
 				   "    -v                   Verbose output\n" + 
+				   "    --bundled-header     Do not attempt to include 'mono-config.h'. Define the entry points directly in the generated code\n" +
 				   "\n" + 
 				   "--simple   Simple mode does not require a C toolchain and can cross compile\n" + 
 				   "    --cross TARGET       Generates a binary for the given TARGET\n"+