mkosi: Store /etc under /usr/share/factory/mkosi

If we're making a /usr only image, we still want to populate /etc
fully on first boot. To make that possible, let's copy /etc to
/usr/share/factory/mkosi in a finalize script, which runs after
all changes to the image have been made. Let's also add a tmpfiles
snippet that merges /usr/share/factory/mkosi with /etc on boot to
populate /etc.

mkosi.presets/20-final/mkosi.extra/usr/lib/tmpfiles.d/99-mkosi.conf

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+C+! /etc - - - - /usr/share/factory/mkosi

mkosi.presets/20-final/mkosi.finalize

@@ -0,0 +1,4 @@
+#!/bin/sh
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+cp --archive --recursive --no-target-directory --reflink=auto "$BUILDROOT"/etc "$BUILDROOT"/usr/share/factory/mkosi