sysupdate: include default component for feature-all

sysupdate: include default component for feature-all

--component-all is documented to include the default component-less
installation. Do not drop it merely because the context operates on a
root/image, or because all its transfers are currently disabled by
features.

This lets --component-all --feature-all enable-feature write the
default component feature drop-ins instead of succeeding with no
components selected.

TEST-72-SYSUPDATE covers both all transfers disabled by features and
the same default component feature operation under --root=.

Repro: create a default feata.feature plus a transfer gated by feata,
then run:
build/systemd-sysupdate --root="$root" --component-all --feature-all enable-feature

Before: no drop-in was written.

Follow-up for: 4481661a75acc01b5d66aa36443a7b80b557e4ba