sign-tfm-image: Add new function for signing TF-M images

A new CMake module (SignTfmImage.cmake) is introduced
to add the needed sign function to be used to sign
the non-secure (application) binaries.

This change enhance re-usability and decrease code
duplication within the existing applications and applications
to be added.

Signed-off-by: Ahmed Ismail <Ahmed.Ismail@arm.com>