rauc-hawkbit-updater: add support for provisioning

Read-only Oniro images now have space to provision rauc-hawkbit-updater
configuration file. The startup of the update service is conditional on
the presence of that file. The default configuration file is stored as
an example in the read-only image.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>