With the image 2.7b4 we changed from using ext3 to using ext4 for the root file system.
The U-Boot environment must be changed to take that into account, but, to not override settings you might have in the environment we do not reset it automatically.
If you do not want to keep the environment set it to the new defaults as follows:
env default -a
saveenv
If you want to keep your environment, first print the two affected variables for your records, then set them to their new values:
printenv emmcargs sdargs
env default emmcargs sdargs
saveenv