20-grub-install: Explicitly check '+debug' suffix for debug kernels
    
    The kernel-install script is also used to install kernels when built from
    source using the `make install` target.
    
    And if this source contains modifications, a '+' is added as suffix by the
    scripts/setlocalversion if no LOCALVERSION was set in the kernel config.
    
    This confuses the grub2 kernel-install plugin, since it currently assumes
    that any kernel that contain a version with a '+' suffix is a debug kernel.
    
    But the match is too greedy, just having '+debug' should be enough to check
    whether the kernel to install is a debug kernel or not.
    
    Resolves: #2148351
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>