rtos: Introduce cmsis-config and freertos-config

Introduce the `INTERFACE` targets
* `cmsis-config` for the configuration of CMSIS components
* `freertos-config` for the FreeRTOS kernel

Onto those targets, a user application can apply compile-time macros or
include paths to its configuration headers.

The advantages of using configuration targets are:
* the target names are readable - they indicate that their purposes are
  for setting configurations
* when multiple components (e.g. CMSIS RTX, CMSIS DSP) share one way of
  configuration, the user only needs to set the configuration once
* documentation is simpler and easier to make sense of

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