Feature, cellular, SARA-R5: add support for CellTime. (#955)

CellTime, the feature through which accurate hardware timing may be achieved through the cellular network and/or GNSS, is now supported: see u_cell_time.h and the uCellTime API.  This feature is supported on SARA-R5 cellular modules only.

Documentation update only: editorial change to DATABASE.md.

Cellular change only: re-entrant versions of file list functions. (#724)

Re-entrant versions of the file list functions, uCellFileListFirst_r(), uCellFileListNext_r() and uCellFileListLast_r(), are added.  These are required for the up-coming HTTP Client API.

Documentation change only: correct link to DIY porting instructions.

AT client fix only: remove asyncRunningMutex in uAtClientIgnoreAsync(…

…). (#530)

The function uAtClientIgnoreAsync() had a mutex, asyncRunningMutex, which it would lock while doing its work in order to guarantee that, if it were called while an asynchronous task was currently in progress, it would wait for that asynchronous event to complete before returning, ensuring that none were even partially executing. However that was a mistake: the things called by the asynchronous event will have their own mutex protection and so, for instance in the case of the cellular API and URC-related events, the following sequence could occur during shut-down of the cellular API:

- task (A): calls one of the cellular API shut-down functions (uCellRemove() or uCellDeinit()), which locks the cellular API mutex gUCellPrivateMutex.
- task (B): starts running an asynchronous event and so asyncRunningMutex is locked while executing it.
- task (A): as part of the cellular API shutdown, calls uAtClientIgnoreAsync() which wants to lock asyncRunningMutex but is blocked as task (B) has it.
- task (B): the function called by the asynchronous event is in the cellular API and so wants to lock gUCellPrivateMutex but is blocked as task (A) has it: WE ARE LOCKED.

With this commit the asyncRunningMutex in uAtClientIgnoreAsync() is removed; it is up to the things being called asynchronously to provide their own mutex protection (which they do) and that will already ensure that such calls complete in an organised way; asyncRunningMutex is not actually necessary.

Test automation change only: re-enable instance 10. (#433)

Instance 10 was misbehaving, the HW has now been replaced and hence it is re-enabled.

 doc: Add note about STM32CubeIDE project being replaced with Makefil…

…e (#369)

Heads up for our users that we are about to remove the STM32CubeIDE
project from the ubxlib repo.

The message is embedded as HTML in readme_images/important_msg.svg.
GitHub have very restricted inline HTML support so there seems to be no
way of changing background color of things. But by using a .svg file and
the "foreignObject" it is possible.

Merge commit: add support for cellular SARA-R422

Documentation change only: update link to port directory

Feature addition: MQTT client. (u-blox#183)

MQTT client API, implemented and tested currently on cellular SARA-R5 only.