kernel: Reduce peak disk usage during build

Since commit f620b1b693ec25af, the build directory is located inside the
source directory. Thus, the `cp -dpR` copies gigabytes worth of .o files
only to be deleted later on when we trim all non-essential files from
`$dev/lib/modules/${modDirVersion}/source/` thus causing a significant
amount of wasted I/O and peak disk usage.

As `cp` doesn't come with a `--exclude` flag, use rsync. And throw out
the Documentation folder while at it.

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