Platform/DeveloperBox: create shared .DSC include file

We are going to add a separate .DSC/.FDF combo for the standalone
MM components. So put all the pieces we will share in an include
file that both .DSC files can include.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>