Current build system assumes same CPU arch on secure and non secure side.
This task is to cleanly isolate secure side build from non secure side build.

Objectives:
a. separation of cmake files - top level configuration files are currently common across secure and non secure build. The configurations must be separated out between secure and non secure builds. Proposed tree:

tf-m/ConfigXXX.cmake -- only configure the bootloader and secure side build.
tf-m/app/ConfigXXX.cmake --- config files for non-secure file build