aemfvp-a-rme: Change build to include distro packing.

Modify build-buildroot.sh to update the kernel, grub and the initrd
in the build. This accelerates build time by removeing the need to
boot the distro and install the packages. The system will do the
following:

1. Download the correct version of the Debian image.
2. Remove the filesystem from the Debian image.
3. Upgrade the kernel within the filesystem.
4. Update the initrd to match the new kernel.
5. Repack the updated filesystem in to the image.
6. Build the update image into the buildroot filesystem.

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
Change-Id: Ic9c36c10e4acb2169c5e81c66985fe247a76a877