Add support for bare dm-crypt partitions

[anatol]

booster currently supports encrypted partitions of LUKS format. LUKS is essentially a header that contains password information + dm-crypt data.

Add support for bare dm-crypt partitions. The key information and mounting options will be located somewhere else - either at other partitions (e.g., EFI) or other devices such as USB.

[nsajko]

In case it helps, there are three different key file location cases here that mkinitcpio supports:

1. key file is on a block device, possibly with an offset
2. key file is on a path on a file system on a block device
3. key file is on a path on the initramfs