xtensa: build similar to other Zephyr arches

* CONFIG_SOC is now properly set and we do not need a separate
  XTENSA_CORE build variable

* Some unnecessary macro -D CFLAGS in the Xtensa Makefile removed

* There is no default SOC selection, it is now done explicitly in
  the board's defconfig

* CONFIG_<board name> now renamed to CONFIG_SOC_<board name in
  uppercase> to conform to established style.

Issue: ZEP-1711
Change-Id: I88997530db09970b7fdd1c3e3d355bfca9d0be1a
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>