Add support for keyfiles
Created by: lemmarathon
cryptkey boot parameter enables keyfile decryption, which may be helpful for unlocking via USB key or for having to type the passphrase only once when using GRUB cryptodisk.
If the path is omitted,
cryptkey will attempt to use
/crypto_keyfile.bin by default.
If keyfile decryption fails, init will fall back to passphrase mode.