Allow GetChallenge at any position during TA execution

This commit contains newly generated code (model repo
rev:64873c79081922a7969f6ed01d16efb04911cdbf).

Previously GetChallenge was only allowed by according states before the
certificate verification process and after MSE:SetAt. The new
implementation allows GetChallenge at any position during TA trhoug
usage of history states.

processCommandGetChallenge now needs to generate a new challenge
regardless of the previous challenge state.

processCommandExternalAuthenticate now needs to check the existence of a
challenge and throws SW 6985 if no challenge was generated yet.