Draft: Add kernel variant to the JSON Index

With this field, desktop installers like the UBports Installer can list different kernel variants without having to do fuzzy searching.

This is probably the worst possible way to achive this (lol).

Fixes: #154

bpo/api/job_callback/build_image.py

@@ -98,7 +98,7 @@ def upload_new_files(path_temp, files):
     return f"got {count} file(s)"
 
 
-def upload_finish(session, image, path_temp, dir_name):
+def upload_finish(session, image, path_temp, dir_name, kernel):
     # Create target dir
     path = bpo.images.path(image.branch, image.device, image.ui, dir_name)
     os.makedirs(path, exist_ok=True)
@@ -123,7 +123,7 @@ def upload_finish(session, image, path_temp, dir_name):
 
     # Generate HTML files (for all dirs in the images path, including the path
     # of this image and its potentially new parent directories)
-    bpo.ui.images.write_index_all()
+    bpo.ui.images.write_index_all(kernel)
 
     # Start next build job
     bpo.repo.build()
@@ -137,6 +137,7 @@ def job_callback_build_image():
     device = bpo.api.get_header(request, "Device")
     dir_name = get_dir_name(request)
     job_id = bpo.api.get_header(request, "Job-Id")
+    kernel = bpo.api.get_header(request, "Kernel")
     ui = bpo.api.get_header(request, "Ui")
 
     session = bpo.db.session()
@@ -148,4 +149,4 @@ def job_callback_build_image():
 
     if files:
         return upload_new_files(path_temp, files)
-    return upload_finish(session, image, path_temp, dir_name)
+    return upload_finish(session, image, path_temp, dir_name, kernel)