always unconditionally create /dev/rtc and use it internally

Partially revert 2b3c81b02fa5dd47b19558c7684e113f36a48486, which
tried to avoid inconsistent rules about when and how to create the
/dev/rtc symlink.

Instead of conditionally or not creating the /dev/rtc link at all,
now always create it with additional and more reliable udev rules.

First try to find the "system rtc" with the hctosys flag, if this
is not found, fall back to create the link for /dev/rtc0.

Our code now never actively searches for the "system rtc" it can
always use /dev/rtc.