Preserve $HOME in cape setup script

See machinekit/machinekit#328

Using `su` clobbers the `$HOME` variable, which breaks X authentication and perhaps location of `machinekit.ini` as well.

Here, `su` isn't necessary, since `sudo` already obtains the elevated privileges needed to write to `bone_capemgr.*/slots`.

This patch may apply to other cape setup files as well.