Fix breaking OTA CBMC proofs

[AniruddhaKanhere]

Description

This PR fixes the broken CBMC proofs for the below two functions.

• Posix_OtaReceiveEvent
• Posic_OtaSendEvent

It does so by introducing a stub for the use of poll function introduced with https://github.com/aws/ota-for-aws-iot-embedded-sdk/pull/433. The stub non-deterministically returns either a success or a failure along with corresponding flag(s) being set.

Checklist:

• [x] I have tested my changes. No regression in existing tests.
• [x] My code is formatted using Uncrustify.
• [x] I have read and applied the rules stated in CONTRIBUTING.md.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[AniruddhaKanhere]

These changes were incorporated in https://github.com/aws/ota-for-aws-iot-embedded-sdk/pull/446.