Since commit f620b1b693, 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.
29b84841d2