architecture: rework to integrate add-ons

[joelguittet]

The purpose of this task is to rework the architecture of the mender-mcu-client in order to permit the future development of add-ons (Device Configuration, Device Monitoring, etc).

Add-on will be optionally integrated based on compilation config. This permit each project to limit the size of the mender-mcu-client library while having the minimum set of functionalities required.

For this purpose the current mender inventory implementation will be moved to an add-on (well indeed it's highly recommended to have it anyway!)

Tasks includes:

• separation of the mender-client "update" support and mender-inventory functionality
• creation of the mender-inventory add-on
• rework of tasks to limit the memory consumption due to increasing number of tasks for each add-on (work-queue will be used instead)
• general cleanup and update of the examples, update of the documentation