Drop 10_linux_bls and avoid corner case of blsdir set with ostree
    
    The logic to parse the BLS configs to generate a set of menuentry commands
    that's needed on ppc64le machines with bootloaders that don't have support
    to parse BLS config directly, was implemented in a 10_linux_bls script.
    
    But there's no need to have a separate script just for this and this logic
    can be merged into the 10_linux script to avoid code duplication.
    
    Also since the blscfg module will also now be used by ostree-based distros
    there is a possible corner case in which a user set the blsdir variable to
    a BLS directory path that is different than the default used by ostree.
    
    So to avoid possible issues, only drop the marker file to specify that the
    bootloader has support to parse BLS files if this variable hasn't been set.
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>