Fix mechanism for OTA upgrade from Sming 4.2

[mikee47]

Further to discussion in #2727 this PR addresses issues arising when attempting to migrate from a Sming 4.2 application using OTA only.

• Once we have a good path for migrating from 4.2 via OTA, update documentation and include any appropriate code improvements

[what-the-diff]

PR Summary

• Fix operator bool() method in the PartitionTable class
  Now returns the opposite of whether "mEntries" is empty, which is the intended behaviour. This means that the operator will show if there are items in the collection instead of indicating whether it's empty.

• Allow FullType casting for Device::Type
  Supports building partition tables in code

• Document
  Describe method for installing partition table on legacy devices via OTA.