samples: rework show-tuple

Now that we are using defconfig files, the samples do not contain
the full configuration, so we can not simply parse them to show
their content.

Instead, we must fake recalling a sample, and parse the generated
.config file.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 #!/bin/sh

 set -e

 printf "Running autoconf...\n"

 autoconf -Wall --force

 printf "Done. You may now run:\n    ./configure\n"