Notice for apt upgrade:
apt upgrade on R01 os will mess up the content of extlinux.conf
lead the boot failed
so when this happens
mount the sd card(with R01 os) on a PC (like Linux )
then go into the boot partition of the sd card
you will see
extlinux.conf
extlinux.conf.cpi.bak
replace extlinux.conf with extlinux.conf.cpi.bak
the extlinux.conf.cpi.bak is for this kind of situation