third_party/freertos: Allow global freertos dir
third_party/freertos: Allow global freertos dir

Adds build-time errors in the FreeRTOS GN build to detect when FreeRTOS
is in use, but not properly set up. This allows projects to now set the
dir_pw_third_party_freertos build argument globally without breaking
toolchains that do not use FreeRTOS (and therefore haven't properly set
it up).

Change-Id: I993053b32d9ddbd1529966349932374525686a55
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/72962
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Commit-Queue: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed