target: add option to set the root password

Add an option in the menuconfig to specify a root password.

If set to empty, no root password is created; otherwise, the password is
encrypted using MD5 (MD5 is not the default for crypt(3), DES-56 is, but
MD5 is widely available, not-so-strong, but not-so-weak either).

Add a check for 'mkpasswd' as a new dependency.

[Peter: fix typo/capitilization and simplify logic]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>