kdump-lib: attempt to fix BOOT_IMAGE detection
    
    Resolves: bz2174836
    Upstream: Fedora
    Conflict: None
    
    commit f9c32372d2c8d5e58024d2ddc0b70498c696b5d8
    Author: Dusty Mabe <dusty@dustymabe.com>
    Date:   Wed Jun 22 12:34:12 2022 -0400
    
        kdump-lib: attempt to fix BOOT_IMAGE detection
    
        Currently $boot_img can get bad data if running on a platform
        that doesn't set BOOT_IMAGE in the kernel command line. For
        example, currently:
    
        - s390x Fedora CoreOS machine:
    
        ```
        [root@cosa-devsh ~]# sed "s/^BOOT_IMAGE=\((\S*)\)\?\(\S*\) .*/\2/" /proc/cmdline
        mitigations=auto,nosmt ignition.platform.id=qemu ostree=/ostree/boot.0/fedora-coreos/2a72567ac8f7ed678c3ac89408f795e6ccd4e97b41e14af5f471b6a807e858b9/0 root=UUID=2a88436a-3b6b-4706-b33a-b8270bd87cde rw rootflags=prjquota boot=UUID=f4b2eaa5-9317-4798-85cf-308c477fee4c crashkernel=600M
        ```
    
        where on a platform that uses GRUB we get:
    
        - x86_64 Fedora CoreOS machine:
    
        ```
        [root@cosa-devsh ~]# sed "s/^BOOT_IMAGE=\((\S*)\)\?\(\S*\) .*/\2/" /proc/cmdline
        /ostree/fedora-coreos-af4f6cc7b9ff486cfa647680b180e989c72c8eed03a34a42e7328e49332bd20e/vmlinuz-5.18.5-200.fc36.x86_64
        ```
    
        We should change the setting of the boot_img variable such that it will
        be empty if BOOT_IMAGE doesn't exist.
    
        With this change on the s390x machine:
    
        ```
        [root@cosa-devsh ~]# grep -P -o '^BOOT_IMAGE=(\S+)' /proc/cmdline | sed "s/^BOOT_IMAGE=\((\S*)\)\?\(\S*\)/\2/"
        [root@cosa-devsh ~]#
        ```
    
        This change mattered much more before the change in c5bdd2d which changed
        the following line from [[ -n $boot_img ]] to [[ "$boot_img" == *"$kdump_kernelver" ]].
        Still I think this change has merit.
    
        Signed-off-by: Dusty Mabe <dusty@dustymabe.com>
        Acked-by: Coiby Xu <coxu@redhat.com>
    
    Signed-off-by: Lichen Liu <lichliu@redhat.com>