buildroot

arch: add Atom CPUs as Silvermont Architecture target

The old Atom target is not really fitting for recent Atom CPUs based
on Silvermont, Airmont or Goldmont. Those have more in common with
older Desktop CPUs than old Atoms.

Signed-off-by: Norbert Lange <norbert.lange@andritz.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>