cmake: build modules under /modules

As we start adding more modules, all of them end up in
${CMAKE_BINARY_DIR}/ and this is getting too busy. We have a module
directory created for each module and the interesting build data is
getting lost in the crowd.

Move all module into ${CMAKE_BINARY_DIR}/modules to and keep them under
one directory.

Future enhancement: maintain the same structure of the modules as
checked out by west.

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