staging: board: description for CONFIG_STAGING_BOARD

A paragraph to describe what CONFIG_STAGING_BOARD is for,
to help developers have better understanding.

Signed-off-by: Song Chen <chensong_2000@189.cn>
Link: https://lore.kernel.org/r/1608889958-32118-1-git-send-email-chensong_2000@189.cn


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>