OvmfPkg: require self-signed PK when secure boot is enabled

OvmfPkg: require self-signed PK when secure boot is enabled

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2506

In all DSC files that define SECURE_BOOT_ENABLE, opt-in into requiring
self-signed PK when SECURE_BOOT_ENABLE is TRUE.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Rebecca Cran <rebecca@bsdio.com>
Cc: Peter Grehan <grehan@freebsd.org>
Cc: Sebastien Boeuf <sebastien.boeuf@intel.com>
Signed-off-by: Jan Bobek <jbobek@nvidia.com>
Reviewed-by: Sean Brogan <sean.brogan@microsoft.com>
Acked-by: Jiewen Yao <jiewen.yao@intel.com>