templates: Filter out POSIX locale for translation

templates: Filter out POSIX locale for translation

The POSIX locale is default or native operating system's locale
identical to the C locale, so no translation to human speaking languages
are provided. For this reason we should filter out LANG=POSIX as well as
LANG=C upon generating grub.cfg to avoid looking up for it's gettext's
message catalogs that will consequently result in an unpleasant message:

  error: file `/boot/grub/locale/POSIX.gmo' not found

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>