cryptsetup-generator: introduce basic keydev support

cryptsetup-generator: introduce basic keydev support

Dracut has a support for unlocking encrypted drives with keyfile stored
on the external drive. This support is included in the generated initrd
only if systemd module is not included.

When systemd is used in initrd then attachment of encrypted drives is
handled by systemd-cryptsetup tools. Our generator has support for
keyfile, however, it didn't support keyfile on the external block
device (keydev).

This commit introduces basic keydev support. Keydev can be specified per
luks.uuid on the kernel command line. Keydev is automatically mounted
during boot and we look for keyfile in the keydev
mountpoint (i.e. keyfile path is prefixed with the keydev mount point
path). After crypt device is attached we automatically unmount
where keyfile resides.

Example:
        rd.luks.key=70bc876b-f627-4038-9049-3080d79d2165=/key:LABEL=KEYDEV