ARM: stub out makebootable so we can build a full haiku-image

src/bin/makebootable/platform/u-boot/Jamfile

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+SubDir HAIKU_TOP src bin makebootable platform u-boot ;
+
+BinCommand makebootable : makebootable.cpp ;

src/bin/makebootable/platform/u-boot/makebootable.cpp

@@ -0,0 +1,6 @@
+int
+main()
+{
+	return 0;
+}
+

src/tools/makebootable/platform/u-boot/Jamfile

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+SubDir HAIKU_TOP src tools makebootable platform u-boot ;
+
+BuildPlatformMain <build>makebootable : makebootable.cpp : $(HOST_LIBSTDC++) ;