freertos: Convert components to INTERFACE libraries

FreeRTOS contains many optional components which are application
specific. We can't reuse the same static library for multiple
applications that all require different optional FreeRTOS components and
configuration. The correct approach here seems to be to use INTERFACE
libraries so FreeRTOS kernel components are completely rebuilt for every
application depending on them. This avoids conflicts when multiple
executables require different components and configuration.

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