Remove buildroot submodule

Apparently, submodules don't work correctly when both `branch` and
`shallow` are specified.  It seems `git` fetches the latest commit and
_then_ tries to switch branches, which always fails:

> Unable to find refs/remotes/origin/2025.08.x revision in submodule
> path 'buildroot'

Let's just get rid of the submodule and do a shallow clone of
`buildroot` manually during the build.

.gitmodules

@@ -1,9 +1,3 @@
-[submodule "buildroot"]
-    path = buildroot
-    url = https://gitlab.com/buildroot.org/buildroot.git
-    branch = 2025.08.x
-    shallow = true
-
 [submodule "aimee-os"]
     path = aimee-os
     url = https://git.pyrocufflink.blue/AimeeOS/aimee-os.git

ci/Jenkinsfile

@@ -34,6 +34,31 @@ pipeline {
             }
         }
 
+        stage('Prepare') {
+            steps {
+                checkout(
+                    poll: false,
+                    scm: scmGit(
+                        branches: [[name: 'refs/heads/2025.08.x']],
+                        browser: gitLab('https://gitlab.com/buildroot.org/buildroot/'),
+                        extensions: [
+                            [
+                                $class: 'RelativeTargetDirectory',
+                                relativeTargetDir: 'buildroot',
+                            ],
+                            cloneOption(
+                                shallow: true,
+                                depth: 1,
+                            ),
+                        ],
+                        userRemoteConfigs: [[
+                            url: 'https://gitlab.com/buildroot.org/buildroot.git',
+                        ]],
+                    )
+                )
+            }
+        }
+
         stage('Build') {
             steps {
                 sh '. ci/defconfig.sh'