Kbuild: Additional default configuration for x86.

This commit adds specific default configuration for various
x86 processors:
 - microkernel/nanokernel atom
 - microkernel/nanokernel minute ia
 - microkernel/nanokernel pentium 4

Change-Id: Iafa0eb29a1ee2e3b467dfe1e1f9ddb587200101e
Signed-off-by: Juan Manuel Cruz <juan.m.cruz.alcaraz@linux.intel.com>