Should be worth trying (since your 'root' you don't need the 'sudo'):
**Back up the passwd and shadow files first** (copy them into /var/local maybe?)
Code:
mount /dev/mmcblk0p1 /tmp/main
chroot /tmp/main
passwd root
Presuming that the passwd command is available
But you can also set 'root' in etc/passwd to not require a password with a text editor (you should have busybox vi available).
REPEAT:
I don't have this firmware version to look at, but
root:!: ..... Should be 'locked' (no shell login)
root:: ...... Should be 'no password required'
root:*: .... Should indicate encrypted password stored in etc/shadow file