Rename `obsolete_ctx` to `unused_ctx` as it is simply unused.
1. Use atomic operations to increase and decrease counter. 2. Style fixes. Remove unused function declaration.
Make the platform context a global variable, adding the refernce counter to it.
1. Move the `mbedtls_platform_context` to be platform code, in `features/mbedtls/platfrom/`. 2. Add static refernce counter, to setup and teardown the platform code only once. 3. Adjust Cryptocell porting accordingly.