This patch adds a new environment variable to the ubuntu-minimal
element called DIB_UBUNTU_KERNEL that allows you to specify the kernel
meta package that will be using to install the kernel inside the image.
It supports "linux-image-generic" (The default), "linux-image-kvm", and
"linux-image-virtual".
This allows building images that are smaller in size (~200MB smaller
qcow2) that have only the kernel modules necessary for virtual
machines.
Change-Id: I8ce65e1d357d15e8ed8995ad1dcaea02bbd1986f
