.gitlab-ci.yml: store .config files as artefacts for defconfig tests

.gitlab-ci.yml: store .config files as artefacts for defconfig tests

Add Buildroot's own .config file, as well as any package's .config file
(uclibc, linux, and busybox), for later inspection should a build fails,
notably due to changes in the kconfig-package infrastructure.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>