merge-tfm-images: Add new function for merging images

A new CMake module (MergeTfmImages.cmake) is introduced
to be used to merge Bootloader, TF-M secure, Non-secure
user application, secure and non-secure provisioning
images into one image to be loaded inside the ROM rather
than loading different images at their respective addresses.

CI is modified to archive the merged binary instead of
separate binaries which would be used the test stage of
the CI.

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>