Add ext4 component

[dumsum]

Ports lwext4 to LionsOS.

• Implements the FS protocol.
• Implementation closely follows the fatfs component.
  • opportunity to factor out some common helper libraries (flagging for later)
• Mostly using default configuration for now.
• Supports all the core operations: file I/O, directory ops, mount/unmount, etc.
• Adds journal/recovery.
• Not implemented for now: symlinks, hard links, file metadata writes (permissions, ownership, timestamps etc.), device nodes
• Only a single worker thread: lwext4's blockdev lock/unlock API, used to protect some internal state, is unimplemented. Needs more investigation on potential concurrency issues before expanding.

FileIO example has been updated to use an ext4 filesystem by passing FS=ext4 (fat still defaults).