lrdp-v2: look for vmlinux in autodump dir when missing

It's not uncommon for the vmlinux file to live in the same directory as
the actual RAM dumps. If the users doesn't explicitly specify a vmlinux
image to use, look for it in the autodump dir. If we still can't find it
there, error out as usual.

Change-Id: I702c5584477e26912e0857dbd03b02491bd33c6c