grubby-bls: print the absolute kernel and initramfs images paths
    
    The BLS file "linux" and "initrd" fields expect a relative paths from the
    root of the partition that contains the kernel and initramfs. The grubby
    tool was printing the value of thse fields and not their full path.
    
    So if there is a partition and /boot is a mount point, he BLS would have:
    
    linux /vmlinuz-4.18.0-38.el8.x86_64
    initrd /initramfs-4.18.0-38.el8.x86_64.img
    
    And grubby would print:
    
    kernel="/vmlinuz-4.18.0-38.el8.x86_64"
    initrd="/initramfs-4.18.0-38.el8.x86_64.img"
    
    But the old tool used to print the full paths of the images, so should be:
    
    kernel="/boot/vmlinuz-4.18.0-38.el8.x86_64"
    initrd="/boot/initramfs-4.18.0-38.el8.x86_64.img"
    
    Resolves: rhbz#1649778
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>