ext: include headers based on HAL being used

For CMSIS we now have HAS_CMSIS which needs to be added to the
SoC definition.
Instead of changing the main Makefile we now include a sub-Makefile
with all related header and library paths that are hosted in
ext/

Move redifintion of LIB_INCLUDE_DIR later to get variables defined
in Makefile.toolchain.*

Change-Id: I9f90f90247c2a66b4565427b89d4e1d4dd5c9622
Signed-off-by: Anas Nashif <anas.nashif@intel.com>