ostree.config: add bootloader config option

Ability to set the bootloader backend that OSTree should use. NB:
normally this should be set to `none` since in modern distros and
bootloaders the BLS is used and the BLS snippets are generated on
`none` but none of the of the specific bootloader tools are run,
like `grub2-mkconfig` for grub.

Update the fedora image manifest to use that config setting.

test/data/manifests/fedora-ostree-image.json

@@ -870,7 +870,8 @@
             "repo": "/ostree/repo",
             "config": {
               "sysroot": {
-                "readonly": true
+                "readonly": true,
+                "bootloader": "none"
               }
             }
           }

test/data/manifests/fedora-ostree-image.mpp.json

@@ -282,7 +282,8 @@
             "repo": "/ostree/repo",
             "config": {
               "sysroot": {
-                "readonly": true
+                "readonly": true,
+                "bootloader": "none"
               }
             }
           }