Comment # 14 on bug 1136977 from Daniel Molkentin

I was jumping to conclusions too fast here. In fact there is a cmdline
scriptlet that purposefully exports $(getenv root=) to $root, see line 133 of
the bootlog:

[    5.116958] dracut-cmdline[211]: /bin/dracut-cmdline@27():
root=live:CDLABEL=openSUSE_Tumbleweed_KDE_Live

So you CAN rely on $boot, but ONLY in the context of cmdline scriptlets.