rename(disk_layout): Remove the legacy use of "legacy"
rename(configure_bootloaders): Remove the legacy use of "legacy"
feat(disk_util): Switch to btrfs for root filesystem in /usr images

This is all that is required to swap out ext4 for btrfs.
No btrfs user space tools or auto-resize support yet.
Merge pull request #183 from marineam/btrfs

Switch to btrfs