base-files: For sysfixtime use hwclock if RTC available

On systems that have an RTC prefer it to the file-based time fixup (i.e.
use hwclock when there is a permanent clock instead of the faked up time
logic that is needed when there is not RTC).

We can't rely on hctosys kernel feature either as we're usually using
RTC as kernel modules which are usually being loaded after hctosys was
run, leading in the following error:

  hctosys: unable to open rtc device (rtc0)

Signed-off-by: Daniel Dickinson <openwrt@daniel.thecshore.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>

SVN-Revision: 48661