install/sd-encrypt: prefer /etc/crypttab with x-initrd.attach over /etc/crypttab.initramfs
Let's get rid of the custom /etc/crypttab.initramfs file in favour of
specifying all encrypted devices in /etc/crypttab.
The x-initrd.attach
option prevents systemd from parsing the crypttab
entries in late userspace and also allows for a simple way to filter them
for inclusion in the initramfs.