auto/config: preserve germinate-output directory when removing config

Germinate doesn't take very long at all to run but downloading the
indices it operates on can take a while and nothing else in auto/config
does so not doing it every time you run "lb config" can be a real time
saver.

The code that invokes germinate already checked if the output was
already there but it was unconditionally deleted by the time control got
to that point.

debian/changelog

@@ -1,3 +1,10 @@
+livecd-rootfs (2.765) UNRELEASED; urgency=medium
+
+  * auto/config: preserve germinate-output directory when removing config to
+    speed up local iteration.
+
+ -- Michael Hudson-Doyle <michael.hudson@ubuntu.com>  Tue, 24 May 2022 12:42:10 +1200
+
 livecd-rootfs (2.764) jammy; urgency=medium
 
   [ Michał Sawicz ]

live-build/auto/config

@@ -1,7 +1,14 @@
 #! /bin/sh
 set -e
 
-rm -rf config
+if [ -e config/germinate-output ]; then
+            mv -T config/germinate-output germ-tmp
+            rm -rf config
+            mkdir config
+            mv -T germ-tmp config/germinate-output
+else
+        rm -rf config
+fi
 
 echo "Building on $(hostname --fqdn)"