angstrom/conf: Add systemd to DISTRO_FEATURES

OE-Core wants systemd to be in distro features
to leverage the systemd infra from Core

secondly in OE-Core there is not build time dep on
update alternatives see 56a59ef12936dcc6464cf1d43dda6957a5aa8c65
update-alternatives now needs to ask for opkg
we were wrongly asking for update-alternatives-cworth
which is a runtime package.

Signed-off-by: Khem Raj <raj.khem@gmail.com>