boot: extension: Move overlay apply custom logic to command level

boot: extension: Move overlay apply custom logic to command level

The extension_overlay_cmd environment variable approach is specific to
the U-Boot extension_board command, while other boot flows (pxe_utils,
bootstd) handle overlay loading differently.

Move the extension_overlay_cmd execution out of the core extension
framework to the command level. This decouples the framework from
command-specific behavior and prepares for future extension support
in other boot flows.

Signed-off-by: Kory Maincent (TI.com) <kory.maincent@bootlin.com>
Reviewed-by: Simon Glass <sjg@chromium.org>