pxe.install: Adding warning to keep kernel in sync

When building the PXE initrd, it's important to boot it with the
associated kernel. Some can get caught by mixing old kernels with newer
initrd leading to incapacity of booting.

This patch adds a simple message to avoid this like :
 ### Build complete ###
 ### initrd is now available at /var/lib/debootstrap/install/D7-F.1.0.0/pxe/../initrd.pxe
 ### Don't forget to use the initrd with its associated kernel : /var/lib/debootstrap/install/D7-F.1.0.0/base/boot/vmlinuz-3.2.0-4-amd64