ArmPlatformPkg: Create include file for default key content.

ArmPlatformPkg: Create include file for default key content.

This commits add file which can be included by platform Flash
Description File. It allows to specify certificate files, which
will be embedded into binary file. The content of these files
can be used to initialize Secure Boot default keys and databases.

Signed-off-by: Grzegorz Bernacki <gjb@semihalf.com>
Reviewed-by: Sunny Wang <sunny.wang@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>