fdisk /dev/mmcblk0
# Delete the second partition /dev/mmcblk0p2
d
2
# Create a new primary partition and use default sizes prompted. This will then create a partition that fills the disk
n
p
2
enter
enter
# Save and exit fdisk:
wNow reboot
reboot
Once rebooted
resize2fs /dev/mmcblk0p2
Your main / partition should be the full size of the disk now.