pytest: add sandbox test for "extension" command

This commit extends the sandbox to implement a dummy
extension_board_scan() function and enables the extension command in
the sandbox configuration. It then adds a test that checks the proper
functionality of the extension command by applying two Device Tree
overlays to the sandbox Device Tree.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
[trini: Limit to running on sandbox]
Signed-off-by: Tom Rini <trini@konsulko.com>