More fixes for BLS
    
    Add some fixes for BLS parsing logic and also make 20-grub.install script
    to query the relative path of the kernel and initramfs images, so BLS can
    also work when /boot is not a mount point or is a btrfs subvolume.
    
    Also pull some build fixes.
    
    Resolves: rhbz#1588184
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>