[powerpc64,ppc64el] ensure fb modules are included in installer images (Closes: #1037186)

Despite the udebs including the required ast driver for many POWER platforms, the module was not included in the final installer images. Make the fb-modules udeb mandatory for ppc64 and ppc64el systems, as these systems typically have on-board framebuffer graphics available.

Signed-off-by: Timothy Pearson tpearson@raptorengineering.com