configs/imx7d-sdb: Add new defconfig

Introduce imx7d-sdb_defconfig that allows booting imx7d-sdb
board using U-Boot and kernel mainline instead of the
vendor provided versions.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
[Thomas: add DEVELOPERS entry, regenerate .gitlab-ci.yml]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>