tf-m: Improve iotsdk_tf_m_sign_image function

Copy tfm_s.elf instead of tfm_s_signed.bin file to build
directory for the user's convenience.
The tfm_s_signed.bin file it is not used anywhere.
The tfm_s.elf will be more useful for debugging purposes.

Signed-off-by: Artur Tynecki <artur.tynecki@arm.com>