package: install bash-completion files only with bash

package/systemd, package/kmod, package/util-linux: don't install
bash-completion files if bash is not selected to be installed.
Otherwise, install bash-completion files.

package/systemd: don't install zsh-completion files is zsh is not
selected to be installed. Otherwise, install zsh-completion files.

package/util-linux: don't install empty
/usr/share/bash-completion/completions directory if bash is not going to
be installed, and install bash-completion files if bash is selected to
be installed.

Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>