fix(vm_image_util): Package up /usr for pxe images.
fix(vm_image_util): Disable installing oem-pxe

It only installed /usr/share/oem/oem-release which we no longer use.
fix(oem/ami): Fix uploading AMI id files, broken by a55f566b.
fix(vm_image_util): Fix building squashfs for /usr

Taking a bit of a new approach to booting PXE images here for both
amd64-generic and amd64-usr. Instead of requiring the user to specify
squashfs and tmpfs on the kernel command line we can simply provide
defaults in the initrd's fstab.
feat(vm_image_util): Rework the qemu wrapper script to work for PXE

Now the script can be used with a disk image or a kernel/initrd.
Using a disk with the PXE kernel should work too but haven't tried it.
fix(vm_image_util): Fix typo in README
Merge pull request #202 from marineam/pxe

PXE images