/e/n/i: move sourcing of /etc/network/interfaces.d/ to begin of file

netcardconfig doesn't properly handle this section and it disappears
when configuring network devices then.

Rewriting netcardconfig is on our agenda (see
grml/grml-network#11), until we get there
let's make sure the status quo works as good as possible.

Take over wording based on what ifupdown's postinst script uses.

components/9990-grml-networking.sh

@@ -39,7 +39,11 @@ fi
 
 # config for loopback networking
 cat > $IFFILE << EOF
-# Initially generated on boot by initramfs
+# Initially generated on boot by initramfs,
+# interfaces(5) file used by ifup(8) and ifdown(8)
+
+# Include files from /etc/network/interfaces.d:
+source /etc/network/interfaces.d/*
 
 auto lo
 iface lo inet loopback
@@ -135,10 +139,6 @@ EOF
     echo>> $IFFILE
 done
 
-echo '# Support overriding defaults:' >> $IFFILE
-echo 'source /etc/network/interfaces.d/*' >> $IFFILE
-echo>> $IFFILE
-
 # dns bootoption
 if [ -n "$DNSSERVERS" ]
 then