OvmfPkg/AmdSev: add Grub Firmware Volume Package

OvmfPkg/AmdSev: add Grub Firmware Volume Package

This is used to package up the grub bootloader into a firmware volume
where it can be executed as a shell like the UEFI Shell.  Grub itself
is built as a minimal entity into a Fv and then added as a boot
option.  By default the UEFI shell isn't built but for debugging
purposes it can be enabled and will then be presented as a boot option
(This should never be allowed for secure boot in an external data
centre but may be useful for local debugging).  Finally all other boot
options except grub and possibly the shell are stripped and the boot
timeout forced to 0 so the system will not enter a setup menu and will
only boot to grub.  This is done by copying the
Library/PlatformBootManagerLib into Library/PlatformBootManagerLibGrub
and then customizing it.

Boot failure is fatal to try to prevent secret theft.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3077
Signed-off-by: James Bottomley <jejb@linux.ibm.com>
Message-Id: <20201130202819.3910-4-jejb@linux.ibm.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
[lersek@redhat.com: replace local variable initialization with assignment]
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
[lersek@redhat.com: squash 'OvmfPkg: add "gGrubFileGuid=Grub" to
 GuidCheck.IgnoreDuplicates', reviewed stand-alone by Phil (msgid
 <e6eae551-8563-ccfb-5547-7a97da6d46e5@redhat.com>) and Ard (msgid
 <10aeda37-def6-d9a4-6e02-4c66c1492f57@arm.com>)]