If the Gaisler toolchain is missing, suggest the add-on.

Demo_C_RTEMS_Gaisler/build-script.sh

 #!/bin/bash
 
 if [ ! -e /opt/rtems-4.10 ] ; then
-    echo The Gaisler RTEMS4.10 toolchain must be placed under /opt/rtems-4.10.
+    echo '[x] The Gaisler RTEMS4.10 toolchain appears to be missing.'
+    echo '[x] Use ~/tool-src/add-ons/install-gaisler-4.10.sh to install it.'
     exit 1
 fi
 export PATH=/opt/rtems-4.10/bin:$PATH