Improve SimpleTrustZone CI and add external build templates

[ReinhardKeil]

• add Native CMake and Zephyr West integration templates
• support GCC and non-secure-only SimpleTrustZone CI builds
• validate application output during FVP execution
• improve FVP UART output, termination, and simulation limits
• clean up retargeting and GCC configuration
• document prebuilt secure-image usage
• align example and template documentation

[jkrech]

LGTM. See comments. I am not clear on whether we still need to build one or more context rather than using an active target (optionally including @set).

[jkrech]

I would expect this to work as well:

cbuild SimpleTZ.csolution.yml --active AVH-NS-only@Debug --toolchain AC6

Unless the toolchain changes in between.

[jkrech]

documented.