This adds the suit_worker_try_prepare / suit_worker_trigger_prepared
pair for using the SUIT worker, and breaks suit_handle_manifest_buf out
of suit_handle_url (where the latter now calls the former).
As part of factoring out reaping of the zombie worker thread, a locking
error that deadlocks the SUIT worker in case the race between the mutex
being unlocked and the thread being reaped hits the necessary handling
code is fixed (and mutex_unlock is called in the error path).
The SUIT CoAP code can make use of the generic subtree handler. This
commit removes the obsolete code to make use of the more generic
nanocoap subtree handlers.
When using SUIT over a link-layer that supports larger PDU, a larger
CoAP blocksize is desirable to speed up firmware downloads.
To support this, make the CoAP blocksize used by SUIT configurable.
