nilrt.conf: set IMAGE_LINGUAS for en-us
nilrt.conf: set IMAGE_LINGUAS for en-us

Currently, images are built with the default IMAGE_LINGUAS?="en-us
en-gb" setting in image.bbclass. We have no need for en-gb, but we would
like en-us.ISO-8859-1, in order to facilitate spot checking execution of
LANG=L1, etc. So override this in distro.conf so that it applies for any
image that doesn't set IMAGE_LINGUAS itself.

Signed-off-by: Richard Tollerton <67c688e7f7482ba5c4ebe3d70bc77594748182a6@ni.com>
Natinst-Reviewboard-ID: 109197
Natinst-CAR-ID: 507802
Reviewed-by: Ken Sharp <e6c8ad8f54d15df4645e8aaf3f7a209bc3688570@ni.com>

ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt