.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>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 1a78211..9e2b111 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -57,10 +57,12 @@
         when: always
         expire_in: 2 weeks
         paths:
+            - .config
             - build.log
             - output/images/
             - output/build/build-time.log
             - output/build/packages-file-list.txt
+            - output/build/*/.config
 
 .runtime_test: &runtime_test
     # Running the runtime tests for every push is too much, so limit to