Work around read only rtc time by using persistently stored time delta

During boot dsme synchronizes system time and rtc time. If the rtc time
can't be modified, the logic misfired so that system time was reset to
whatever time was available via rtc.

Now, if RTC_SET_TIME ioctl fails, dsme stores system time vs read only
rtc time delta in a cache file.

During the next dsme startup rtc time + cached delta time is used to
evaluate what system time should be and further synchronization attempts
from rtc time to system time are skipped.