system: add ability to pass additional users tables.

A new entry has been added to the "System Configuration" menu to allow
the user to set the location of additional user tables (besides the ones
defined in packages).

A user table is a text file, formatted using the mkusers syntax, which
describes the users on the target system, with their UID/GID, home
directory, password, etc.

The target root file system will be populated according the content of
these files.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
[yann.morin.1998@free.fr: use plural TABLES; we need to remove the
    intermediate users_table file, as it is no longer generated in
    one shot, in case a previous run failed and did not remove it]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>