Are boot.img, initrd, and vmlinuz adequate or should the entire contents be hashed? I am not familiar enough with the Linux boot process to answer this with confidence myself.
The safe bet would be to hash all files on /boot (the kernel, system map, MBR and the boot loader configuration) - however who is to say the evil maid won't simply just replace the hashes? So now you need to implement GPG so you can sign & verify the hash file. But what if the evil maid subverts GPG?? Arghhh ...
So, I suggest if you're really worried about that evil maid, a better solution is to keep /boot on a removable keychain type USB device on your belt. It's only needed to boot after all. I use a nifty one on an elastic tether thingy.