integral-image-export.bbclass: Add additional error checking and the initramfs to the exported files.

I haven't yet tested the initramfs stuff but I'm sure it works :)
Imposing some order on the items in the manifest allows us to know
which entries are for which files (rootfs image, kernel, initramfs)
so that when we generate a xen config file for the VM we can guess
at the right values.
integral-image-ndvm-pkg.bb: Add files to SRC_URI and FILES only if file is in the manifest.

Also include the manifest itself.