scripts/mkusers: allow users with no password value set

The following allows a user definition to specify that a created user
entry should not have a password value set. Original implementation
allowed a user definition to provide a password value of "=" (no quotes)
to generate a crypt-encoded empty string value. In some cases, it may be
desired to have no value specified for a user's password. By using a
value "-" for a password, no value will be set in the shadow value.

An example when this can be used is when logging into a terminal.
Logging into a session with an encoded empty password will prompt a user
to enter a password since it does not know the password is empty. If the
password field blank, a login session will not prompt for a password.

Signed-off-by: James Knight <james.knight@rockwellcollins.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>