Disable non-TF-M examples when TF-M is enabled

The memory layout depends on whether TF-M is enabled or not. Currently
MCU-Driver-HAL globally sets a linker script for all executables without
letting executables override it, so we need to disable non-TF-M
applications when TF-M is enabled.

In the future, each executable should be able to specify its linker
script rather inheriting it from the HAL.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>