pxe: Adding USB deployment mode (embedded)

This patch add the USB deployment mode which aims at providing a
bootable usb device that is able to do the deploiement without any
edeploy server.

This solution works like the following:
- building an img image with EMBEDDED_OS & EMBEDDED_ROLE variable
- boot this image on the host to deploy
- the hw matching is done localy
- the target OS is deployed from the USB key to the host machine

Detailed description:
EMBEDDED_OS variable shall point to an existing .edeploy file.
This OS will be included inside the USB bootable image.

EMBEDDED_ROLE variable shall point to the hardware description without
any .cmdb/.spec/.configure extension.
The three configuration file (.cmd/.spec/.configure) are copied on the USB device.

At boot time, the upload.py is executed from the USB device instead of
the edeploy server and uses the cmdb, configure & spec file from the USB
stick.

Those files will remain on a writable partition of the USB stick making
it consistent over time. That way, if you provide a CMDB with several
host to deploy, the same key can be used several time to deploy the
remaining hosts.

A typical make command will be:
        make img EMBEDDED_OS=/var/lib/debootstrap/install/D7-H.1.1.0/deploy-D7-H.1.1.0.edeploy EMBEDDED_ROLE=/home/erwan/Devel/edeploy/config/kvm-usb

The resulting file looks like :
        initrd.pxe-D7-H.1.1.0-with-deploy-D7-H.1.1.0.img

The -with-<OS>.img extension, it's pretty easy to know what is the builtin OS inside this initrd image.