[patch] make doxygen failed

Hi all.
I recently tried to build doxygen target in order to build kernel
sources documentation but failed. The most recent error you would get
trying to build this target is that make tries to change dir to
sys/doc directory. Here are the patches which corrects the build of
doxygen target. You may try it on your own.

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Alexander Mogilny     | The Power to Serve! |
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