The new file programs/demo_common.sh contains initialization code,
utility functions and cleanup code meant to be used by all demo
scripts written in sh.
Initial features:
* msg: Display a message.
* run, run_bad: Run a command, visibly.
* $root_dir, $programs_dir: location of the mbedtls source tree.
* $files_to_clean: files that are cleaned up on exit.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
