shields: scripts: cmake: use list_shields.py in shields.cmake

The logic to "guess" shield names/dirs was duplicated between
list_shields.py (which is used by e.g. west shields) and shields.cmake.
This commit moves the logic to list_shields.py, and updates
shields.cmake to call the script and process its JSON output.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>