BugLink: https://launchpad.net/bugs/1625222

Ubuntu builds the uas and xhci-plat-hcd usb drivers as modules, these
module are needed for d-i to use usb in the installer on the merlin platform.
Signed-off-by: Craig Magina <craig.magina@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>