build: Separate out prebuilt kernel logic from toplevel

Arch-specific stuff shouldn't be in the toplevel Makefile,
forthcoming patches will soon introduce special interrupt
handling logic for other arches.

Change-Id: Ib5a86cbdf5b32ecfce9e5e234f7cbea2bc3ce9c1
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>