clarify KERNEL_PATH

[gaionim]

KERNEL_PATH is not in default config,
it is set in common.sh as
KERNEL_PATH="$INSTANCE_HV_kernel_path"

INSTANCE_HV_kernel_path is read by ganeti-image

if I create instance, no boot partition ( as expected )
but if I run
/usr/share/ganeti/os/image/tools/make-dump
KERNEL_PATH is null, so my dump fails because no boot partition.

If I set KERNEL_PATH in my .conf the dump works.

Regards, Mauro