CI: split out kconfig check and only run when kernel package has changed (!738)

.gitlab-ci.yml

@@ -69,7 +69,6 @@ aports-static:
     - .gitlab-ci/install_pmbootstrap.sh pytest
   script:
     - apk -q add git
-    - su pmos -c "pmbootstrap kconfig check"
     - su pmos -c ".gitlab-ci/run_testcases.sh
         -m 'not pmaports_upstream_compat'"
     - su pmos -c ".gitlab-ci/check_changed_aports_versions.py"
@@ -81,6 +80,21 @@ aports-static:
       - pmbootstrap.cfg
     expire_in: 1 week
 
+# kernel kconfig check
+kernel-kconfig:
+  stage: first
+  only:
+    refs:
+      - master
+      - merge_requests
+    changes:
+      - device/linux-*/config-*
+      - main/linux-*/config-*
+  before_script:
+    - .gitlab-ci/install_pmbootstrap.sh pytest
+  script:
+    - su pmos -c "pmbootstrap kconfig check"
+
 # MR settings
 # (Checks for "Allow commits from members who can merge to the target branch")
 mr-settings: