install: improve bootname search
install: improve bootname search

Look for rauc.slot=<bootname> in addition to root=<device>. This allows
falling back to simple root=/dev/* in the simple cases.

Signed-off-by: Jan Luebbe <9f5a27cdf1a1d6dd5e7372203be5c3a13524cbc8@debian.org>

rauc/rauc,jluebbe/rauc,ejoerns/rauc,jluebbe/rauc,rauc/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc,ejoerns/rauc