system: remove DES password encoding

DES is long dead, it is insecure as hell, and virtually all known
crypt(3) implementations now all support at least md5.

Besides, the character-space of DES-encoded passwords are a sub-set
of the character-space for a clear-text password, so we can't easily
differentiate between the two. Since we're going to change the root
password prompt to support setting encoded passwords (as well as
clear-text passwords), we can't keep DES or we'd be unable to decide
whether we'd need to encode the password or not.

Remove DES encoding altogether (and add a legacy entry). The default is
still md5, and thus there's no backward-compatibility 'select' to add.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Lorenzo Catucci <lorenzo@sancho.ccd.uniroma2.it>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>