Properly introduce selective build options (#625)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/625

Refactor options to do selective build.
* `EXECUTORCH_SELECT_ALL_OPS` by default `ON`, will be set to `OFF` if any of the other options is on.
* `EXECUTORCH_SELECT_OPS_LIST` by default `OFF`
* `EXECUTORCH_SELECT_OPS_YAML` by default `OFF`.

This allows these options to be used outside of selective build examples.

Reviewed By: digantdesai

Differential Revision: D49935141

fbshipit-source-id: b19134b196f5d967da35c64062371745e4530c76