testing: Update base image to Debian jessie

Several packages got renamed/updated, libgcrypt was apparently installed
by default previously.

Since most libraries changed we have to completely rebuild all the tools
installed in the root image.  We currently don't provide a clean target in
the recipes, and even if we did we'd have to track which base image we
last built for.  It's easier to just use a different build directory for
each base image, at the cost of some additional disk space (if not manually
cleaned).  However, that's also the case when updating kernel or
software versions.

testing/scripts/function.sh

@@ -17,6 +17,7 @@
 export TERM=xterm
 RED=$(tput setaf 1)
 GREEN=$(tput setaf 2)
+YELLOW=$(tput setaf 3)
 NORMAL=$(tput op)
 
 # exit with given error message
@@ -66,6 +67,13 @@ echo_failed()
 	echo -e "${RED}$1${NORMAL}"
 }
 
+# write yellow status message to console
+# $1 - msg
+echo_warn()
+{
+	echo -e "${YELLOW}$1${NORMAL}"
+}
+
 # log an action
 # $1 - current action description
 log_action()