Can you boot from uSD?
If so, mount emmc, (lsblk to get right device node),
go to /mount-point/etc, edit shadow (you will need to be root, on uSD boot)
each line is an entry, fields are separated by ":"
the 2nd field is hashed password, about 60+ chars
so, for root what you want (for no password)
root:: .....and so on
IOW, make the 2nd field empty
If so, mount emmc, (lsblk to get right device node),
go to /mount-point/etc, edit shadow (you will need to be root, on uSD boot)
each line is an entry, fields are separated by ":"
the 2nd field is hashed password, about 60+ chars
so, for root what you want (for no password)
root:: .....and so on
IOW, make the 2nd field empty