grubcfg: Use shortProductName.

This is particularly useful if the full product name contains
"GNU/Linux" or "Linux", because e.g. "Generic GNU/Linux GNU/Linux" does
not make sense.

src/modules/grubcfg/main.py

@@ -67,5 +67,5 @@ def run():
     partitions = libcalamares.globalstorage.value("partitions")
     root_mount_point = libcalamares.globalstorage.value("rootMountPoint")
     branding = libcalamares.globalstorage.value("branding")
-    distributor = branding["productName"]
+    distributor = branding["shortProductName"]
     return modify_grub_default(partitions, root_mount_point, distributor)