build: enable shields dir treatment

In dts.cmake, build shield list. For each shield, check if
associate KCONFIG is enabled and if it is, add matching files
to following new cmake variables:
-DTC_SHIELD_OVERLAY_FILE
-DTC_SHIELD_FIXUP
Then, consume these variables when needed.

Shield overlay is conceived as the very first overlay applied
to the board dts. It intends to build a new board that should
then behave as any other board versus common or user's overlay.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>