checkout only a sub-folder in android source

I have downloaded the android sourcs from:
repo init -u http://android.git.kernel.org/platform/manifest.git -b android-4.0.1_r1

I had compiled the sources and changed sources under the external directory.

How do I sync uo my sources so that it is still the same as original sources of android-4.0.1_r1 but also without osing the compiled binaries?

Thanks

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