Add support for crypto keys from external devices

I was experimenting with unlocking my laptop with a small SD card. This just extends the cryptkey syntax, it is very simplistic and could be improved, but I wanted some feedback before first (maybe it is bad idea or other tools solve this problem already).

• cryptkey=UUID=4da61b20-0b3a-5abz-8107-7egdfd047d57: looks for /crypto_keyfile.bin inside UUID=4da61b20-0b3a-5abz-8107-7egdfd047d57.

• cryptkey=UUID=4da61b20-0b3a-5abz-8107-7egdfd047d57/keys/laptop_cryptokey.bin: looks for /keys/laptop_cryptokey.bin inside UUID=4da61b20-0b3a-5abz-8107-7egdfd047d57.

If the given device is encrypted, it will prompt for a passphrase.