Add support for image encryption
Adds support for making isos with encrypted rootfs.
It can be enabled through the encryption boolean in profiledef.sh.
Because it also requires user to add a few hooks and install a slightly modified encrypt hook
(provided by cryptsetup-archiso),
a new encrypted profile has been cloned from releng.
Note: this branch starts from !216.
PS: I don't like the idea to having to replace cryptsetup's encrypt hook,
but I've tried adding a custom hook without results. Further commits are welcomed.