postfix: create or update aliases.db when using systemd

aliases.db should be created by postinstall script, but failed since
queue_directory is not includes root dir ${D}:
    ------
    |newaliases: fatal: chdir /var/spool/postfix: No such file or directory
    ------
initscript will recall newaliases before start postfix daemon, the similar
method, which run aliasesdb to create aliases.db when using systemd, is
introduced to fix this issue.

Signed-off-by: Roy.Li <rongqing.li@windriver.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>