OvmfPkg: Add BUILD_SHELL flag for IA32, IA32X64, X64

OvmfPkg: Add BUILD_SHELL flag for IA32, IA32X64, X64

Add BUILD_SHELL flag, similar to the one in OvmfPkg/AmdSev,
to enable/disable building of the UefiShell as part of
the firmware image. The UefiShell should not be included for
secure production systems (e.g. SecureBoot) because it can be
used to circumvent security features.

The default value for BUILD_SHELL is TRUE to keep the default
behavior of the Ovmf build.
Note: the default for AmdSev is FALSE.

The BUILD_SHELL flag for AmdSev was introduced in b261a30c900a8.

Signed-off-by: Oliver Steffen <osteffen@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>