postfix.inc: fix start postfix failed while hostname is numeric

While hostname is numeric, start postfix failed
...
root@qemux86-64:~# hostname 1.2.3.4
root@qemux86-64:~# systemctl restart postfix
|Job for postfix.service failed because the control process exited
with error code. See "systemctl status postfix.service" and
"journalctl -xe" for details.

root@qemux86-64:~# systemctl status postfix -l
Dec 02 08:05:40 1.2.3.4 aliasesdb[535]: /usr/sbin/postconf: fatal: unable to use my own hostname
Dec 02 08:05:41 1.2.3.4 aliasesdb[535]: newaliases: warning: valid_hostname: numeric hostname: 1.2.3.4
Dec 02 08:05:41 1.2.3.4 postfix/sendmail[537]: warning: valid_hostname: numeric hostname: 1.2.3.4
Dec 02 08:05:41 1.2.3.4 aliasesdb[535]: newaliases: fatal: unable to use my own hostname
Dec 02 08:05:42 1.2.3.4 postfix[540]: warning: valid_hostname: numeric hostname: 1.2.3.4
Dec 02 08:05:42 1.2.3.4 postfix[540]: fatal: unable to use my own hostname
...

Refer meta/recipes-core/initscripts/initscripts-1.0/hostname.sh in oe-core,
add check_hostname.sh and invoke it before postfix start, if the hostname
is invalid, set "localhost" to main.cf.

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>