In /usr/share/doc/kdump-tools/README.Debian the following text appears:
4. Debug Kernel
You *should* have a debug kernel in order for makedumpfile to process
the vmcore file. Without a debug kernel, the transfer process is
reduced to using "makedumpfile -d 1". Options:
A) If /usr/lib/debug/vmlinux-$(uname -r) exists, kdump-tools will
use that kernel.
B) Explicitly set DEBUG_KERNEL in /etc/default/kdump-tools to
point to your debug kernel.
C) None of the above. makedumpfile will still work, but your
dumpfile will be larger and take longer to save to disk.
However all (?) the default -dbg Debian kernel packages put the kernels
in /usr/lib/debug/boot/:
IMHO it would make sense to change (A) so that things worked automagically.
If someone had a truly custom kernel, then tweaking DEBUG_KERNEL per (B)
would make sense, but the 'standard' Debian tools should work with the
standard Debian kernels.
The other options would be to remove the boot/ from the debug kernel path.