Add larger version of ipxe-boot.qcow2 image

This image can be used as the base for baremetal instances with a
flavor that has a disk size of 0, which means the image's virtual
size will be used as the base disk size.

The original image is left unchanged so it can still be used for
instances where a disk size of less than 41 GB is desired.

ipxe/README

@@ -0,0 +1,13 @@
+IPXE images and tools
+---------------------
+
+This directory contains IPXE images and tools for building those images.
+If you are an OVB user you are probably most interested in the pre-built
+images, ipxe-boot.qcow2 and ipxe-boot-41.qcow2.  The former is a minimal
+version of an image that only contains the necessary bits to PXE boot a
+cloud instance.  The latter image is the same, except its virtual size has
+been changed to 41 GB so it can be used as the base image for instances whose
+flavor bases the instance root disk size on the image size.
+
+There is also a diskimage-builder element and a makefile that can be used to
+rebuild the ipxe-boot.qcow2 image.  Simply run ``make`` to build the image.