Partially revert 2b3c81b0, 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.