Raspberry pi 4 support for otaclient. (#182)
* rpi_boot module for rpi4 boot control support,
* boot_control.common: new OTAStatusFilesControl and SlotMountHelper classes, which are prepared for refined
boot_control implementation framework in an effort of reducing duplicated code and unify implementation. Now ota_status
files control logic and slot mounting logic are abstracted and split from the detailed platform specific implementation,
become standalone shared classes.
* common: implement replace_atomic function.
* test files for new boot_control.common.
* test files for rpi_boot module.
* other minor related changes.
