kernel: configs: improve distro.config, mostly for systemd

* add missing options required/optional for systemd
http://cgit.freedesktop.org/systemd/systemd/tree/README


Some options were enabled by default in arch/arm64/defconfig, but not in
arch/arm/multi_v7_defconfig..

* rework/move the options to separate out systemd configs

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>