freertos-cmsis-rtos: Simplify target selection

The freertos-cmsis-rtos target always requires freertos-kernel and
cmsis-rtos-api. Simplify the target selection for the user by linking
the required targets to freertos-cmsis-rtos. The user application can
now just link the freertos-cmsis-rtos target along with their desired
heap management implementation.

The freertos-cmsis-rtos target can only build if cmsis-rtos-api is
available. Add a check for cmsis-rtos-api target being defined before
adding freertos-cmsis-rtos.

Signed-off-by: Robert Walton <rob.walton@arm.com>