POST: Remove duplicated post_hotkey_pressed() functions

This patch introduces a weak default function for post_hotkey_pressed(),
returning 0, for boards without hotkey support. The long-running tests
won't be started on those boards. This default function was implemented
in many board directories. By implementing this weak default we can
remove all those duplicate versions.

Boards with hotkey support, can override this weak default function
by defining one in their board specific code.

Signed-off-by: Stefan Roese <sr@denx.de>