test templates: m3ulcb-nogfx: use core-image-minimal

test templates: m3ulcb-nogfx: use core-image-minimal

The -nogfx build uses the core-image-minimal target, not the
agl-demo-platform build target, so ensure the NBD rootfs filename is
the right one.

Change-Id: I81fa24a55cfa6acfbeda987f31d6dca5fea9dd52
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/10119
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>