fs/common: allow custom user table to override package-defined users

fs/common: allow custom user table to override package-defined users

Currently, when a custom user table and a package define the same user,
the settings from the package takes precedence over the ones from the
custom user table.

However, it makes sense to allow the settings from the custom user table
take precedence. For example, it would allow redirecting the user's
home directory to an alternate location (e.g. away from tmp and into a
partition that is persistent).

The support/scripts/mkusers script will only retain settings from the
latest definition it finds.

Thus, by passing the custom user table after the package defined users,
it is possible to override the package provided user definitions.

Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>