Don't try to switch to a BLS config if GRUB_ENABLE_BLSCFG is already set
There's no point on executing the script if GRUB_ENABLE_BLSCFG has already
been set. Currently was checking if an user explicitly set it to false to
avoid enabling the BLS configuration, but it should also be avoided if was
already set to true by a previous package update or during installation.
Signed-off-by: Javier Martinez Canillas <email@example.com>