The kbuild infrastructure in 2.5.21 uses the filename to generate a
dependency file, which is passed into gcc using -Wp,-MD,filename.
Unfortunately, the ',' in the filename is interpreted by GCC as an
argument separator for the preprocessor.  So, with a filename
foo,bar.c gcc ends up passing the three arguments "-MD", "foo" and
"bar.c" to cpp, which promptly complains.

We therefore rename these files to use an underscore instead of a
comma.