OvmfPkg: fix dynamic default for oprom verification policy PCD without SB

OvmfPkg: fix dynamic default for oprom verification policy PCD without SB

I missed the following, both while reviewing and while testing commit
6041ac65ae87 ("OvmfPkg/PlatformPei: DENY_EXECUTE_ON_SECURITY_VIOLATION
when SEV is active", 2017-10-05):

If "-D SECURE_BOOT_ENABLE" is not passed on the "build" command line, then
OVMF has no dynamic default at all for
"PcdOptionRomImageVerificationPolicy". This means that the PcdSet32S()
call added in the subject commit doesn't even compile:

> OvmfPkg/PlatformPei/AmdSev.c: In function 'AmdSevInitialize':
> OvmfPkg/PlatformPei/AmdSev.c:67:3: error: implicit declaration of
> function '_PCD_SET_MODE_32_S_PcdOptionRomImageVerificationPolicy'
> [-Werror=implicit-function-declaration]
>    PcdStatus = PcdSet32S (PcdOptionRomImageVerificationPolicy, 0x4);
>    ^
> cc1: all warnings being treated as errors

Make the current, SB-only, 0x00 dynamic default unconditional.

This is the simplest approach, and it reflects the intent of original
commit 1fea9ddb4e3f ("OvmfPkg: execute option ROM images regardless of
Secure Boot", 2016-01-07). Without SECURE_BOOT_ENABLE,
"SecurityPkg/Library/DxeImageVerificationLib" is not used anyway, so the
PCD is never read.

This issue was first caught and reported by Gerd Hoffmann
<kraxel@redhat.com>'s Jenkins CI. Later it was also reported in
<https://bugzilla.tianocore.org/show_bug.cgi?id=737>.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Fixes: 6041ac65ae879389f3ab5c0699f916d3e71c97fe
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
[lersek@redhat.com: trim commit message as suggested by Jordan]
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
[lersek@redhat.com: add reference to TianoCore BZ#737]