debug: move segger configs to subsys/debug

We have dependency on this module in code which is part of Zephyr. When
this module is split out of the tree we need to be able to build. Move
this Kconfig part to be part of zephyr and keep the external code in
ext/ with plan to split it out on the future.

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