root.sh: Provide additional diagnostics when package installation failed

Print information from apt's dependency resolver. This helped to debug the libsensors4 -> libsensors5 transition breaking our self built mesa and thus preventing the image build to succeed.

root.sh

@@ -180,7 +180,10 @@ function setup_locale()
 function install_pkgs()
 {
     chroot "${basedir}" apt-get -y update
-    chroot "${basedir}" apt-get -y install ${packages}
+    if ! chroot "${basedir}" apt -y install ${packages}; then
+	echo "Package installation failed, this might help to diagnose the problem:"
+	chroot "${basedir}" apt -y -o Debug::pkgProblemResolver=yes install -y --simulate ${packages}
+    fi
 }