remove nano/micro defconfigs, use 1 file only

Do not use micro_* or nano_* defconfigs, instead maintain platform
in one single defconfig and merge nano or micro support on top.

Change-Id: I0d5184f37865ed8312e516e48cf5a8584a287dfe
Signed-off-by: Anas Nashif <anas.nashif@intel.com>