OvmfPkg: introduce EnrollDefaultKeys application

OvmfPkg: introduce EnrollDefaultKeys application

Add the OvmfPkg/EnrollDefaultKeys shell application source as it is at the
"edk2-20190308git89910a39dcfd-2.el8" tag in RHEL8's downstream "edk2"
package.

Further patches in this series will replace Red Hat-specific bits in the
application, with a conduit to pass in parameters directly from the host
side, on both QEMU and Xen.

(Note: at the moment, Xen doesn't support Secure Boot, due to lacking a
standards-conformant variable driver stack. However, that could change
soon
<https://fosdem.org/2019/schedule/event/vai_implementing_uefi_variable_services_in_qemu/>,
and then this facility will become useful on Xen too.)

The use case for this application (including why it is only being added to
the DSC files) is explained in detail in
<https://bugzilla.tianocore.org/show_bug.cgi?id=1747#c0>.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien.grall@arm.com>
Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=1747
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Gary Lin <glin@suse.com>