agl-users: factor out reusable include file

agl-users: factor out reusable include file

Refactor the recipe to put the actual useradd configuration into an
include file.  This will allow other recipes that need to use the
agl-driver user for file ownership to pull that in as opposed to
duplicating the configuration and risking mismatches.

Bug-AGL: SPEC-4952

Change-Id: Ica16a13a3750722c6d22b14944898f7b289e4bbe
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/29340
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>