U2/U3: kernel 3.8.x too old for BTRFS

[gangely]

Hello.

The 3.8.x kernel used for the U2/U3 image can not mount BTRFS formatted with 'skinny-metadata' feature.

The is a concern, because this feature is now set by default when formatting with mkfs.btrfs.

To solve this problem, the kernel should be upgraded to at least 3.10.x

As comparison, present kernel for ODROID-C1 is fine.

Many thx for all your work,

Gaetan