Fix builds (bootctl and ostree)

A couple of build fixes:

• Fix bootctl install by truncating /etc/machine-id much later
• ostree: Don't try to drop resolv.conf from an not-existent /etc

scripts/ostree-prepare.sh

@@ -7,6 +7,3 @@ rm -f /usr/lib/tmpfiles.d/home.conf
 
 # tries to create unneeded legacy file
 rm -f /usr/lib/tmpfiles.d/dbus.conf
-
-# Empty etc/machine-id so it's regenerated on first device boot
-echo > /etc/machine-id