testing: Extract and patch each kernel version only once

This allows us to do modifications to the kernel tree and rebuild that kernel
using make-testing. We can even have a git kernel tree in a directory to
do kernel development.

testing/scripts/build-guestkernel

@@ -26,12 +26,14 @@ then
 	execute "wget -q $url"
 fi
 
-log_action "Unpacking kernel"
-execute "tar xJf $KERNELTARBALL"
-
 KERNELDIR=$BUILDDIR/$KERNEL
 cd $KERNELDIR
 
+if [ ! -d "$KERNELDIR" ]
+then
+	log_action "Unpacking kernel"
+	execute "tar xJf $KERNELTARBALL"
+
 	if [ $KERNELPATCH ]
 	then
 		log_action "Applying kernel patch"
@@ -39,6 +41,7 @@ then
 		log_status $?
 		[ $? -eq 0 ] || exit 1
 	fi
+fi
 
 execute "cp $KERNELCONFIG .config" 0