examples: Check RTOS target existence instead of variable

The variables `IOTSDK_CMSIS_RTX` and `IOTSDK_FREERTOS` are for fetching
components. Whether RTX or FreeRTOS is available for use by SDK examples
is better checked by the availability of their CMake targets.

Note: Building different flavors of the same example based on RTOS
availability is for code reuse only. A real user project does not need
to do this if it intends to support one RTOS only.

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