target: add different methods to encode passwords

Passwords can be encoded in different ways (from the weakest
to the strongest): des, md5, sha-256, sha-512

Add a choice entry to select the method, defaulting to 'md5'.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>