buildroot

.gitlab-ci.yml: Update imx6-sabresd defconfig entries

After the conversion to SPL the imx6sabresd defconfig targets have
been renamed, so update their entries accordingly.

Suggested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>