The rtc_drv on x86 can now by used to also set the RTC. If the config
attribute 'allow_setting_rtc' is set to 'yes' the driver will update
the RTC from the content of the 'set-rtc' ROM module. A valid ROM must
contain a top node with the following attributes: 'year', 'month',
'day', 'hour', 'minute' and 'second'.
* Only rudimentary checking of the provided values is done.
* '12H' mode is not supported.
Fixes#3438.